(metricunitscomp.txt, METRICUN.CMP Cp7 000410 OJ)
Olle Järnefors COMPENDIUM
Sibeliusgången 44, 8 tr Version Cp7
S164 76 KISTA Sweden 20000410
+468752 93 25

: Metric Units Galore: 311 Named Units with Symbol, Definition and Size
*********************************************************************
: Abstract: 311 metric units, more or less connected to SI, are
: listed, with English name(s), symbol, definition, and 4 figure
: value in SI units. Both currently used and obsolete units
are included. They are classified into 12 classes after
decreasing metricness. Both existing standards and real use
: have been taken into consideration. Also interesting failed
: unit reforms, current proposals for improvement of SI and
: possible future improvements are covered. Symbol and name
indexes and a reference list are included.
Content 
1. Introduction
1.1. Overview [1]
1.2. Summary of named metric units [1]
1.3. What is a metric unit? [2]
1.4. What is a standard unit? [3]
1.5. Ambiguous unit names [3]
1.6. Special symbols and character replacements in this text [4]
2. Principles for metric unit names and symbols
2.1. SI prefixes [5]
2.2. Unit names and unit symbols [5]
2.3. Unit symbol expressions [5]
: 2.4. Writing unit symbols in restricted character sets
2.5. English names of composite units [6]
2.6. Rulegenerated unit names [6]
3. Official units related to SI
3.1. Class 1: SI base units (8 units) [7]
3.2. Class 2: Derived SI units with official SI names (15 units) [8]
3.3. Class 3: Additional SI names for some SI units (6 units) [9]
3.4. Class 4: Unofficial names for some derived SI units (19 units) [10]
3.5. Class 5: Officially recognized units being powers of 10
of SI units (3 units) [11]
3.6. Class 6: Other officially recognized units related to SI
(6 units) [12]
4. Other metric units
4.1. Class 7: Unofficial units being integral nonzero powers of 10
of SI units (68 units) [13]
4.2. Class 8: Other unofficial units related to SI, which can have
prefixes (55 units) [14]
4.3. Class 9: Unofficial units related to SI which cannot have prefixes
(66 units) [15]
4.4. Class 10: Officially recognized units unrelated to SI (2 units) [16]
4.5. Class 11: Unofficial units unrelated to SI (51 units) [17]
4.6. Class 12: Notoriously ambiguous units (12 units) [18]
Annexes
A. Power of 10 symbol index [A]
: B. Recommended unit symbols [B]
C. Unit symbol index [C]
: D. Unit name index [D]
: E. Proposed new prefixes and SI units [19]
: F. Can prefixes cause ambiguity?
: G. Different unit systems [E]
: H. Time units
: I. Angle units
: J. My 12 proposals for improving SI [I]
: K. Differences between Swedish and English unit names
L. Reference list [F, J]
M. Still to do
N. Document history [G, K, M]
: The content of the square bracket at the end of an entry indicates
: the number(s) of the section(s) (or letter(s) of the annex(es)) in
: which this material was included in earlier versions of this document.

: Original home: The latest published version of this document
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Copyright? Please reuse this information! I will appreciate if
the source is acknowledged.
: Previous knowledge: The reader should know the difference
: between physical quantity and unit, and understand the
 importance of unit conversions (which should have been
 made obvious by the recent spectacular failure of a NASA
 mission to Mars).
Feedback: Send questions, comments, corrections, and
 criticism to the email address .
: Acknowledgements: I am grateful for valuable comments on
: previous versions of this document by Mark Brader (Toronto,
: Canada), Dave Keenan (Brisbane, Australia), H. Peter Anvin
: (Yggdrasil Computing, Inc.), and Amos Shapir
: (GivatHashlosha, Israel).
 Nontrivial changes since last version (version Cp6) are
: marked by "" at the beginning of each line. Other nontrivial
: changes since last published version (version Br2) are marked
: by ":".
: Related documents 
The British/US Unit Mess:

Old Swedish Units:

: 
1. Introduction
================
1.1. Overview

: This document is a description of most named metric units, in
: particular those included in or related to the International
: System of Units, SI. It does not contain information about
: which metric units are actually used for a certain quantity,
: but it describes most named metric units and gives
: sufficient information for the construction and interpretation
: of derived metric units. Given a metric unit, its size and
: exact definition can easily be found in this document or
: deduced from it.
:
: Section 1.3 tries to define what a metric unit is and explain
: which kinds of units are described in this document. Section 1.4
: is about the different kinds of standard metric units. The next
: section is about how unit names can be ambiguous. Various markings
: and replacement symbols I use are explained in section 1.5.
:
: Then follows the technical substance. First, in section 2.1,
: the SI prefix system is defined. The principles for unit names,
: symbols and expressions are stated in the following sections.
: Sections 2.4 and 2.6 contains rules for generating unit names
: for composite or derived units and gives a number of rules for
: the systematic generation of names for certain kinds of units.
:
: The following sections contain the main description of the
: metric units, partitioned into 12 classes of decreasing
: metricness. First, in a table, the symbol, English name,
: exact definition, and physical quantity for which the unit
: is usable are given. For units that cannot be defined using
: only SI units, an approximate value in SI units is also given.
: After that come various notes on alternative symbols, names,
: spellings, ambiguities, combinability with prefixes,
: restrictions to particular application areas and other things.
:
: Finally, a number of annexes are included: prefix symbols in
: alphabetical order, a recommended set of unit symbols, index of
: all units arranged after symbol, index of all units arranged
: after English name, proposed extensions of SI, an analysis of
: the possibilities of ambiguous symbols for multiple units,
: a summary of 11 unit systems, an overview of the nondecimal
: multiple units for time, a short description of my 12 proposals
: for improving SI, a comparison of English and Swedish unit
: names, a reference list, a document history.
:
: The 12 proposals are in summary:
: 1: A name for 1
: 2: A special name for 1 for relative size/change
: 3: Complete general rules for unique unit expressions
: 4: Prefixes for certain powers of 2
: 5: Rename the kilogram
: 6: New names for two thermal units
: 7: Discourage use of nondecimal multiple units
: 8: Remove the symbol ambiguities in ISO 2955
: 9: Allow money units into SI
: 10: Peaceful coexistence between SI and other units
: 11: New prefixes for powers of 10
: 12: Accept only rational additional units with SI
:
: The main character of this document is descriptive: facts
: about units, captured in sections 3 and 4; a description
: of the existing system of rules for metric units and their
: symbols in section 2. Normative material is included in
: annex B (my recommendations for which unit symbols to use)
: and annex J (my thoughts on how SI should be further
: developed). Of a theoretical nature is the conceptual
: analysis of "metric unit" and "standard unit" in sections
: 1.3 and 1.4, and the combinatorial analysis of the
: potentials for ambiguity in different systems of prefix
: and unit symbols, in annex F.
:
: I have mostly neglected the cultural history of units. My
: emphasis is on hard issues of practical importance: The physics
: of units: How can they be defined in physical terms? How big/small
: are they? The logic of units: Which are combinable with prefixes?
: Which names and symbols are ambiguous? Which units form unit
: systems? Which unit symbol systems are consistent?
1.2. Summary of named metric units

: This is a summary of most nonobsolete metric units with
: names of their own, which are described in this document
: (coherent SI units are marked with "*", units allowed together
: with SI with "+", unofficial names of SI units with "="):
:
: absorbed dose: gray*
: acceleration: G
: acoustic impedance: acoustic ohm
: activity: becquerel*
: amount of data: bit=, byte
: amount of substance: mole*
: angle: second+, minute+, gon, degree+,
: radian*, revolution
: area: barn, are
: brightness: magnitude
: capacitance: farad*
: Celsius temperature: degree Celsius*
: charge: elementary charge, coulomb*, faraday(1)
: communication traffic intensity: erlang=
: conductance: siemens*
: dose equivalent: sievert*
: electric current: ampere*
: electric potential: volt*
: energy: electronvolt+, hartree, joule*,
: dietician's calorie
: entropy flow: thermal ohm=
: explosion energy: tonne TNT
: force: newton*
: frequency: hertz*
: frequency interval: cent, octave, decade(1)
: illuminance: lux*
: inductance: henry*
: information theoretic entropy: hartley, shannon
: length: bohr, metre*, astronomical unit, light year,
: parsec, international nautical mile
: linear density: denier, tex
: logarithmic quantities: neper=, bel
: loudness: sone=
: loudness level: phon
: luminous flux: lumen*
: luminous intensity: candela*
: luminosity: solar luminosity
: magnetic flux: weber*
: magnetic flux density: tesla*
: mass: electron rest mass, unified atomic mass unit+,
: kilogram*, tonne+, solar mass,
: metric point, metric grain, metric carat
: mass fraction: karat
: mechanical impedance: mechanical ohm
: molality: molal=
: power: watt*, metric horsepower
: pressure: pascal*
: quantities of dimension 1: 1*
: reactive power: var=
: refractive power: diopter=
: resistance: ohm*
: share: per cent
: signal event frequency: baud=
: solid angle: steradian*
: specific acoustic impedance: specific acoustic ohm
: thermodynamic temperature: kelvin*
: time: second*, minute+, hour+, day+, week,
: month, common year, tropical year,
: Gregorian year, Julian year, sidereal year,
: anomalistic year, leap year, decade(2),
: century, millennium
: velocity: knot
: volume: litre+
1.3. What is a metric unit?

: It is not very clear what should be regarded as metric
: units. An informal definition sufficient for my purposes
: in this document is this: A metric unit can be exactly
: defined in the base units of SI (including the metre) or
: has been defined independently of SI by an authoritative
: worldwide organization in terms of constants of nature.
: For example, the Bohr magneton, defined in terms of universal
: constants, is a metric unit, but old photometric units
: like the HefnerKerze, which were based on specially
: designed sources of light, are not considered metric units.
:
: Most of the units formerly used in Britain and the countries
: of the Commonwealth, many of which are still used in USA,
: are parts of an AngloAmerican tradition which was distinct
: from and hardly at all influenced by the metric units.
: Although the base units of the British and US unit system
: are officially defined in terms of SI units since at least
: 1963, they are not treated as metric units in this document.
:
: What can be called national metric units, i.e. units defined
: as simple multiples of the SI unit for the quantity, but used
: only in a single country, are not covered here. A few boundary
: cases, where the unit is or has been used in a few countries,
: such as the metric hundredweight, the metric pound and the
: Swedish new mile, have been included.
:
: Also worthwhile is to discern the different degrees of
: metricness that characterizes the metric units. I have chosen
: to treat the following criteria, given in priority order, as
: determining the metricness of units:
: a) Is the unit fairly well defined, or is it notoriously
: ambiguous?
: b) Is the unit related to SI (i.e. is it definable in the
: base units of SI) or not?
: c) Is the unit derivable from SI base units, or is it
: a multiple other than 1 of the corresponding SI unit
: (or vice versa), or something else?
: d) Is the unit internationally standardized or not?
: e) Is the unit an integral power of 10 of an SI unit,
: or a unit being some other multiple of an SI unit
: (or vice versa), or something else?
: f) Can the unit have SI prefixes or not?
: g) Is the unit a general unit for quantities of a certain
: dimension, or is it merely a special name for such a
: unit, to be used for one quantity with this dimension?
:
: Combining these criteria, I have defined 12 different classes
: of metric units, which are exhasutive and mutually exclusive.
: In order of decreasing metricness:
: 1) SI base units
: 2) Derived SI units with official SI names
: 3) Additional SI names for some SI units
: 4) Unofficial names for some derived SI units
: 5) Officially recognized units being powers of 10 of SI units
: 6) Other officially recognized units related to SI
: 7) Unofficial units being integral nonzero powers of 10 of SI units
: 8) Other unofficial units related to SI, which can have prefixes
: 9) Unofficial units related to SI which cannot have prefixes
: 10) Officially recognized units unrelated to SI
: 11) Unofficial units unrelated to SI
: 12) Notoriously ambiguous units
I only cover units that have been given a specific English name
: (and in a few cases units that have names in other languages),
not those that always are expressed as a product or quotient of
units, such as W·h (energy) or kg·m/s (momentum). The physical
quantity for which each unit is most frequently used is indicated
within curly brackets in the following tables.
1.4. What is a standard unit?

: Which units are standard? Foremost of them are the SI units,
: the only units which are sanctioned by an international
: convention and part of international law.
:
: The original metre system was adopted by the French National
: Assembly 5 April 1795. It covered length, area, volume and weight
: or mass (a sharp distinction between weight and mass was usually
: not made at that time). The time units, already universal, were
: not reformed.
:
: A decree by Napoleon in 1812 partially retracted the new units.
: From 1840 on, the metric system once again became the only
: legal system in France. The development in Belgium and the
: Netherlands was parallel. The metre system was adopted in
: Prussia in 1868 and then started to spread throughout
: continental Europe. (It was adopted in Sweden and Norway
: in 1875, with a 14 year transition period. Denmark switched
: to metric units in 1910.) The use of metric units was
: legalized in Great Britain in 1864, metric units were adopted
: as the preferred system 101 years later. The transition to
: metric units in USA is anticipated in the Metric Conversion
: Act of 1975, though voluntary, with no fixed time goal, and
: still with little impact in everyday life and nonexporting
: industry.
:
: The responsibility for the further development of the metric
: system was taken over by an international body through the
: International Metre Convention of 20 May 1875, the first
: international convention about scientific matters. SI was
: established in 1960 according to decisions based on this
: convention.
:
: The decisions are taken by CGPM (Conférance Générale
: des Poids et Mesures), which consists of delegates for the
: governments of the nations that are members of the metre
: convention. It meets at least every six years. CIPM (Comité
: International des Poids et Mesures) summons CGPM, makes
: proposals for revisions of SI, and leads the work of BIPM
: (Bureau International des Poids et Mesures) in Sèvres,
: France. This is the international centre for scientific
: metrology which, among other things, is responsible for
: keeping the international kilogram and metre prototypes.
: CIPM is assisted by CCU (Comité Consultatif des Unités).
It is possible to distinguish six categories of units,
given here in order of decreasing degree of standardization.
: Besides the units indicated here for a certain category, also
: their multiple units, formed using the SI prefixes, should be
: included in each category.
A) Units of the International System of Units (SI): These are
the units in classes 1  3. They are defined by CGPM (the
governing body of the international Metre Convention from
1875).
B) Additional units, whose use is allowed within the SI
framework. These are the units in classes 5, 6, 10. They are
also defined by CGPM.
C) Other units defined in the international standard ISO 31:1992
: [D]: These include neper, sone, var (class 4), per cent
: (class 7), bel, phon (class 8). International standards are
: adopted by ISO (International Organization for
: standardization) and IEC (International Electrotechnical
: Commission).
D) Some units whose names and symbols are indicated by ISO
: 31:1992 or related standards [D, E, F, P], although their
: use is not recommended in the standard: bar, gauss, maxwell,
: poise, stokes, tex (class 7), gon, (kilo)gramforce,
(kilo)pond, conventional (milli)metre of mercury,
conventional (milli)metre of water (class 8), metric carat,
metric horsepower, oersted, standard atmosphere, technical
atmosphere, torr (class 9), astronomical unit, parsec
(class 11), International Steam Table calorie, thermochemical
: calorie, 4 degree Celsius calorie, tropical year, (imprecise)
: year (class 12).
: E) Other units whose use earlier has been temporarily accepted
: together with SI units. These include angstrom, are,
: barn, gal, rad (class 7), curie, röntgen (class 8),
: international knot, international nautical mile (class 9).
:
: F) Other units defined by international scientific or
: technological organizations like IUPAP (International Union
: of Pure and Applied Physics), IUPAC (International Union
: of Pure and Applied Chemistry), IAU (International Astronomical
: Union), and ITU (International Telecommunication Union).
: To this category belong the solar mass, the solar luminosity,
: shannon.
: G) Units not defined in open international standards. These
: include other units that are widely used, internationally,
: nationally or in a certain application field, units which
: earlier belonged to categories A  D or F, units which have
: been proposed for international standardization but failed.
1.5. Ambiguous unit names

: All unit names are ambiguous in some way or another (except
: possibly some names of units for quantities of dimension 1).
: One or more of these different types of ambiguity can affect
: a unit name:
: * Fundamental physical ambiguities:
: Any unit for a physical quantity can directly or indirectly
: be traced back to one or more definitions of base units,
: involving the description of a physical situation. Such
: descriptions are by necessity incomplete. For example,
: the definition of the astronomical unit involves the
: mass of the sun as a base unit. That mass decreases slowly,
: however, and the boundary between the sun's atmosphere and
: the interstellar medium is not welldefined. This lack of
: precision (or fuzziness, or ambiguity) may be impossible
: to measure and only of theoretical interest in this case,
: but for other units like the metre, progress in measurement
: technology has made necessary more precise redefinitions.
: Furthermore, because the laws of physics cannot be fully
: known, any physical definition of a unit can miss factors
: which slightly affect the size of the unit.
: * Successively better defined units:
Some basic units have been redefined one or more times
during their history. Normally the new definition has been
more precise than the previous definition. The old value is
then a good approximation of the new value. In that case I
regard the two units as essentially the same, and have given
only the latest definition.
: * Slightly different units:
In other cases there exist different units with the same
(short) name but different definitions, giving slightly and
measurably different values. This is e.g. true for the
"international" ampere and the "absolute" ampere. In these
: cases I have included the most modern unit in the table for
: the unit class in section 3 or 4, and defined the other unit
: in a note. If the other unit is also included in a unit class
: table, its name is made unambiguous by the addition of an
: attribute. Thus, two ampere units are defined in the unit
: class tables, the "ampere" and the "international ampere".
: Distinguishing attributes include: common, conventional,
: international, long, mean, metric, normal, original, reduced,
: regular, short, standard, technical.
:
: * Notoriously ambiguous unit names:
: A more serious difficulty affects the units calorie, month,
: year, and other units defined as multiples of them, which
: are collected in class 12. They come in several somewhat
: different sizes. This can be important or unimportant,
: depending on the context. I have differentiated between these
: variants in this document using additional attributes in the
: unit names and subscripts in the unit symbol, such as for
: International Steam Table calorie, cal_IT, and included these
: more precise units as units in their own right in other classes
: than class 12. In practice, these longer names and symbols are
: usually not used, and it may be difficult or impossible to
: decide from the context which size was intended, even in cases
: where this level of precision is not unimportant. The generic
: names of these units are marked with "[a]" in the tables.
:
: * Units for different quantities with the same name:
: A fourth kind of ambiguity exists when two or more units for
: different quantities have the same name. They are separated
: by a suffix like "(1)", "(2)" etc. (which is arbitrary and
: local to this document).
:
: * Language dependence of unit names:
: Few units have the same name in all languages. Especially
: for old units like hour the names may be totally different
: in different langauges. Also for newer units, variations
: in spelling and plural forms between languages are common.
: It is possible that one string of letters can be the name
: of one unit in some language and the name of another unit
: in another language. The differences between English and
: Swedish for the set of units defined in annex B are
: described in annex K.
:
: The use of unit symbols instead of unit names introduces
: other risks of ambiguity. See section 2.2.
:
: When a unit has several different English names, these are
: included as separate entries in the indexes.
1.6. Special symbols and character replacements in this text

In this document, "=" is used for exact, definitional equality.
"~=" means approximately equal to. I also use "^" to indicate
that the following number is an exponent and "_" to indicate
that the following letters or digits form a subscript. I have
written "H2O", although the "2" should be a subscript digit
in this symbol.
According to international standards either the continental
multiplication sign "·", the middle dot
(character number 00B7 in ISO 10646/Unicode), also used for the
: scalar product of two vectors, or the English multiplication
sign "×" (00D7 in ISO 10646), also used for vector product, may be
 used for multiplication of numbers or units. I use the former.
 On the other hand, following the AngloAmerican convention,
 I use ".", not ",", as decimal point.
: In this text, "/" is used only to denote division, and for
: example not to separate alternatives.
The marking "[o]" after the name of a unit (or prefix) means that
: it is obsolete. "[p]" means that the unit has been proposed but
: not come into common use. "[a]" means that the unit name is
: seriously ambiguous. Capital letters in square brackets refers
: to the litterature listed in annex L. Numbers in square brackets
: at the end of a unit definition refers to a subsequent note.
: The quantity for which a unit is used is indicated in its
: definition within a curly bracket.
I have used these symbols instead of some characters that
ASCII lacks:
: Latin small letter h with bar !?
: <^d> superscript Latin small letter d !?
: <^g> superscript Latin small letter g !?
: <^h> superscript Latin small letter h !?
: <^m> superscript Latin small letter m !?
: <^s> superscript Latin small letter s !?
: Greek small letter alpha (03B1 in ISO 10646)
Greek small letter gamma (03B3 in ISO 10646)
Greek small letter lambda (03BB in ISO 10646)
Greek small letter pi (03C0 in ISO 10646)
Greek small letter sigma (03C3 in ISO 10646)
Greek capital letter Omega (03A9 in ISO 10646)
prime (2032 in ISO 10646)
double prime (2033 in ISO 10646)
: <%.> per mille sign (2030 in ISO 10646)
: <\prime> reversed prime !?
: <\bis> reversed double prime !?
2. Principles for metric unit names and symbols
================================================
2.1. SI prefixes

One of the beautiful properties of SI is the potential for
creating new units of suitable size by using standard prefixes
designating powers of 10. The prefixes are used uniformly with
: almost all SIrelated units. (For time units, prefixes are
: normally used only for units smaller than the second(1). The minute(1),
: hour(1), and day are special nondecimal multiple units for time,
: whose use is officially allowed together with SI units. A
: number of other multiple units for time are also in use. An
: overview of time units is given in annex H, which can be
: contrasted with the simplicity of the decimal prefixes used
: for other quantities than time. The angle unit degree, which
: can be used together with SI units, can be divided into the
: nondecimal multiple units minute of arc and second of arc.
: An overview of these and other units for angle is included
: in annex I.)
The following prefixes are presently defined (although
myria is not included in SI):
10^ symbol name 10^ symbol name
     
24 Y yotta 24 y yocto
21 Z zetta 21 z zepto
18 E exa 18 a atto
15 P peta 15 f femto
12 T tera 12 p pico
9 G giga 9 n nano
6 M mega 6 µ micro [3]
4 my myria[o]
3 k kilo [5] 3 m milli
2 h hecto [4] 2 c centi [4]
1 da deca [1,4] 1 d deci [4]
1 D deca [1,2,4]
[1] "deka" is also sometimes used.
[2] "D" is an obsolete symbol for deca.
[3] A standardized substitute symbol for micro is "u".
: [4] In SI it is recommended that hecto, deca, deci and
: centi are used only in volume units and units derived
: from volume units.
: [5] The capital "K" is sometimes misused as symbol for the
: kilo prefix, especially in literature in the English
: language.
Prefixes can only be attached to unprefixed unit names or
symbols. The application of a prefix has higher precedence
than even the exponentiation operator: 1 dm^3 = 1 (dm)^3
2.2. Unit names and unit symbols

: The standardized units of categories A  C, as well as some
integer powers of 10, have internationally standardized symbols,
consisting of 1  2 letters (powers of 10) or 1  3 letters
(units). The full names of the prefixes and units may vary
between languages, the symbols do not. The symbols are case
sensitive.
: Unit names take plural "s" in English. Units names like "siemens",
: "hertz" and "lux", which in themselves end with an "s" sound,
: have the same form in the singular and the plural. Unit symbols
: are always the same, irrespective of the size of the numerical
: value.
: In SI, unit names are written consistently with small letters,
: even those named after a person. The only exception is
: "degree Celsius". I have followed the same rule for other
: metric units here, with the degree units for temperature as
: exceptions. The SI unit names have no diacritical marks,
: even when they are named after a person whose name contain
: such marks. I have not followed that rule for other metric
: units, but in some case I have noted an alternative
: English diacritfree spelling of a unit name.
:
: Symbols are in use also for most units of categories D  G.
: In a few cases more than one symbol is used for the same unit,
: though, and collisions, where the same symbol are used for
: more than one unit are not uncommon. There are even cases
: were the symbol defined for one unit coincides with the
: name of another unit, such as "rad", symbol for radian but
: also the name of an obsolete radiological unit for absorbed
: dose. Since short unit names are often used in practice in
: the same way as unit symbols, the rad unit for instance should
: be regarded as having "rad" as symbol, besides the standard
: symbol "rd", making "rad" an ambiguous unit symbol. (These
: ambiguities are shown by the symbol index in annex C; probably
: "b" is the most ambiguous of all symbols in use.)
: Especially in English usage many nonstandard short forms are used,
: such as "cc" for cm^3, "gm" for gram and "sec" for second (which
: is also ambiguous with the standard substitute symbol for second
: of arc). These are not covered in this document.
: In a few cases (notably: are, bar, erg, gon, rem, tex, var),
: the unit symbol is identical to the English name. Both
: "The angle is 382 gon." and "The angle is 382 gons." are
: therefore correct.
For disambiguation some unit symbols include subscripts. In
practice these are usually dropped.
The combination of a power of 10 and a unit may result in a
symbol that is already used for another unit. I have noted
: the most important such ambiguities in sections 3 and 4.
: (This issue is analyzed in detail in annex F, which also
: uncovers two mistakes in this regard made by ISO when
: defining substitute symbols for some units to be used
: when a distinction between small letters and capital
: letters is not possible to achieve.) With some units
: prefixes are never used in practice, with others they
: should not be used because of ambiguity problems.
These cases are also noted.
: The unit symbol index in annex C is useful for interpreting
: unit symbols used in existing texts. When writing numerical
: data, one should, in my opinion, use symbols only for the units
: of categories A  E, and use prefix symbols only together with
: these. Other units should be written using their full names.
: The set of usable unit symbols is enumerated in annex B.
2.3. Unit symbol expressions

: Symbols for physical quantities shall, if possible, be written
: with italic letter symbols. They do not indicate any particular
: unit. Symbols for units are always written with upright letters.
: In numerical expressions of quantities, the numerical value is
: written before the unit. They are separated by a space, except
: if any of the angle unit symbols "°", "" or "" is
: used.
:
: When "s" is the symbol of the unit second(1), the natural
: interpretation of an expression like "2/3 s" is "(2/3) s",
: not "2/(3 s)". On the other hand, the natural interpretation
: of "2 m/3 s" is "(2 m)/(3 s)", not "((2 m)/3) s". It is
: nontrivial to formulate general interpretation rules with
: both these as consequences. In this document I have used the
: unambiguous, though clumsy, notation "(2/3) s". I have also
: used subexpressions like "(1 s)" in formulas, instead of
: simply writing only the unit symbol "s", as if it were an
: ordinary symbolic constant.
:
: SI multiple units are as a rule decimal. For time and angles
: nondecimal multiple units are used, though. Mixed numerical
: expressions in these cases can be written as e.g. "23 h 55 min" and
: "59°2508.3". No longer recommended is to use
: expressions like "59°2508.3", or to write angles
: expressed in gons, new minutes and new seconds in the same way,
: using "<^g>" and "<\prime>" and "<\bis>" as unit symbols. Nor
: is it recommended to writes time figures in the same way, using
: "<^d>", "<^h>", "<^m>" and "<^s>" as symbols.
:
: Many units do not have names of their own and are usually
: written using unit symbol expressions. A consequence is that
: some units can be denoted by several, equally adequate,
: expressions. For example the coherent SI unit for momentum
: may be written "kg·m/s" or "N·s", the SI unit for magnetic
: dipole moment can be written "A·m^2" or "J/T".
:
: For SI, the following examples show the recommended ways of
: writing unit expressions:
:
:  Reciprocal units:
: m^1
:
:  Simple products:
: N·m·s or N m s
:
:  Simple quotients:
: mol
:  or mol/s or mol·s^1
: s
:
:  Mixed expressions:
:
: J J
:  or  or J/kg K or J/(kg·K) or J kg^1 K^1
: kg·K kg K
:
: Pa·s Pa s
:  or  or Pa·s/m^2 or Pa s/m^2 or Pa s m^2
: m^2 m^2
:
: The variants least prone to be misunderstood seem to be:
: N·m·s; mol/s; J/(kg·K); Pa·s/m^2.
:
: Especially for electrical units, it is common practice to juxtapose
: symbols of units that are multiplied, e.g. Wbm. This may lead to
: ambiguity in some cases, such as "W/mK": watt/millikelvin or
: watt/(meter·kelvin)? Reciprocal units are sometimes written in the more
: suggestive way of e.g. "23.08/m", instead of "23.08 m^1". Occasionally,
: "×" is used as multiplication operator also in unit expressions.
: According to international standards, it is acceptable in numerical
: expressions but not in unit expressions.
:
: Units derived partly from nondecimal multiple units should generally
: be avoided. Often used, though, among such units are: A·h, kcal/h,
: hk·h, lm·h, km/h, r/min, kW·h, kW·h/a.
: 2.4. Writing unit symbols in restricted character sets
: 
:
: The following rules have been laid down in an international
: standard [E].
:
: In unit expressions, the multiplication sign can be substituted
: by a full stop. Exponents may be written using normalsized
: digits on the line.
:
: When text is produced in a system lacking one or more of the
: following special characters, the substitute symbols indicated
: should be used:
:
: Preferred Substitute Unit/prefix
: symbol symbol name
:   
: µ u micro
: Ohm ohm
: °C Cel degree Celsius
: ° deg degree
: mnt minute of arc
: sec second of arc
:
: (These new symbols do not conflict with the symbols already
: used for prefixes or metric units, so they may be used even
: when the system makes possible the use of the preferred symbols.)
:
: When text is produced in a system also lacking the ability to
: distinguish between small letters and capital letters, the
: following symbols shall be replaced by the special substitute
: symbols indicated. (They are written here with small letters.)
:
: Preferred Substitute Unit/prefix Comment
: symbol symbol name
:    
: E ex exa ev = electronvolt, not exavolt
: P pe peta p = pico
: M ma mega m = milli
: S sie siemens s = second(1)
: a ann year a = ampere
: t tne tonne t = tesla
: AU asu astronomical unit au = atto(unified atomic mass unit)
: pc prs parsec pc = picocoulomb
:
: Other prefix and unit symbols in the set covered by the
: relevant international standard [E] are simply converted
: to small letters (or capital letters), if necessary. These
: include the standard symbols for prefixes (except the recently
: defined Y, y, Z, z), symbols for the SI and other official
: units of classes 1  6 and 10, and the unit symbols:
: a, are, bar, gon, P, St.
:
: The standard mentioned includes a strange provision
: which it is difficult not to characterize as a mistake,
: namely the specification of the same symbol, "a", for two
: different units, the are and the year (unspecified variant).
: Strangely, this ambiguity is supposed to be resolved when
: a monocase system is used, since "a" meaning year is
: to be replaced by "ann", while "a" meaning the unit are
: is to be replaced by "are". Among other things, this makes
: automatic conversion of data using the unit symbol "a" to
: a monocase system impossible in the general case. Without
: mentioning this embarrasing problem, I tacitly assume in the
: rest of this document that the recommended symbol for the
: are unit is "are" and for the year is "a" (with "ann" as
: substitute symbol).
:
: Another valid criticism of this standard is that the
: full set of symbols to be used in monocase systems is
: inconsistent, in exactly two respects:
: (1) "pa" can mean either picampere (pA) or pascal (Pa).
: (2) "pev" can mean either petavolt (PV) or picoelectronvolt (peV).
: For a proof, see annex F.
2.5. English names of composite units

: The English name of a prefix unit or a unit that is expressed as
a product or quotient of other units can be derived according to the
following patterns.
"U", "U1", "U2", and "U3" are the symbols of the units with
the names "unit", "unit1", "unit2", and "unit3", respectively,
: and "x" is the symbol for the multiple (or submultiple) with the
name "prefix":
Symbol Name Note
  
xU prefixunit
U^2 unit squared
U^3 unit cubed
U^4 unit to the fourth power
U^1 reciprocal unit
U1·U2 unit1 unit2
U1/U2 unit1 per unit2
U1/(U2·U3) unit1 per unit2 unit3
m^2 square metre when corresponding to area
m^3 cubic metre when corresponding to volume
xm^2 square prefixmetre when corresponding to area
xm^3 cubic prefixmetre when corresponding to volume
: When combined with a unit whose names start with a vowel,
: a prefix normally loses its last letter, e.g. hectare = 100 are.
2.6. Rulegenerated unit names

: There exist a number of unit name valued functions, i.e.
: rules that will generate the name of a new unit, given a unit name
: or quantity. The most important are:
:
: * = the multiple unit which is
: (the power of 10 indicated by ) times bigger than .
: Example: 1 microyear ~= 32 s.
:
: * atomic unit of = a.u. of = the coherent unit
: for in the atomic unit system, i.e. the unit that is
: derived from the four base units: electron rest mass, bohr,
: elementary charge, reduced Planck constant.
: Examples: 1 atomic unit of velocity ~= 2.19 Mm/s;
: 1 atomic unit of mass ~= 9.11·10^31 kg
: On the other hand: 1 unified atomic mass unit ~= 1.66·10^27 kg
:
: * light