[1.144] Symbols1.144 SymbolsAlways define the units of measure that are used in the document, and their abbreviations and symbols, at the start of the text. Symbols that are widely known (distance, mass, volume, and time—see Standard abbreviations for symbols in common use that do usually not require definition). Repeat these definitions as often as you think is necessary to avoid any possibility of misunderstanding. Even the most widely used symbols, prefixes, and abbreviations are frequently used incorrectly. For example, what is the correct symbol for the kilotonne: kt, Kt, kT or KT?1.145 Symbols, usage
• If the symbol is well-known (°C, %, m, and kg, for example), it can be used without definition (see Units of measure, common symbols). Otherwise, define your symbols at first use and in the list of abbreviations in the front matter.[1.146] Metric system1.146 SI symbols1.147 Punctuation, symbols1.148 Plurals, symbolsNote. The name of the unit is a noun, is not capitalized (except degree Celsius), and obeys the standard rules of punctuation when spelled out.1.149 Case, prefixesSymbols must be written as they are defined no matter where they occur in a sentence or title. Capitalizing a symbol changes its meaning. For example, t is the symbol for the tonne, and T is the symbol for the tesla and the tera prefix.There is a tradition in the metric system that the first (or only) letter of an unprefixed unit symbol is capitalized if (and only if) the unit's name comes from a proper name. Thus W is the symbol for the watt, and A is the symbol for the ampere, because these units are named for scientists. The symbol for litre is an exception, and is normally capitalized. Both L and l are acceptable but L is preferred, because the letter l is easily confused with the number 1. Note that the spelled-out unit is not capitalized (joule, for example) except for degree Celsius and degree Fahrenheit (but you must include the uncapitalized “degree”).
10-9 10-6 10-3 103 106 109 1012 1015 1018 1021Note that the prefix k, although an anomaly (prefixes that signify a factor of greater than 1 are otherwise upper case), is never capitalized, as in kB, km, kg, kPa, and kL.Note. In cases where a non-standard format is in general use and does not cause confusion—kWh, for example—define it as an abbreviation. For more information, see Non-standard units.Note. You will find instructions on how to type a middle dot, non-breaking space, thin space, and other special characters in Typing shortcuts.1.151 Non-standard units
The terms bank cubic metre (in situ volume of rock in m3) and loose cubic metre (broken volume of rock in m3) are widely used in the mining and construction industries. They are also useful when expressing large volumes (1000 m3 = kBCM, not km3). The SI-compliant km3 (= GBCM) is rarely used. [1.152] List of SI symbols1.152 List of SI symbolsThe base, derived, and acceptable SI units (International Organization for Standardization) and their symbols are identified in the following table. Other units shown are outside the SI system but are widely used. For simplicity, treat units that are outside the SI system as symbols (see also Imperial system, below).Refer to Glossaries for symbols and abbreviations that are specific to an industry.
m3 Hz [s-1] kg/m3 m/s2 N [m·kg·s-2] Pa [N·m2] m2 T [V·s/m2] Imperial. Do not abbreviate. Do not confuse with t (tonne) or T (tesla). Usually (US) short ton, but ambiguous: (US) short ton (2000 lb = 907.18 kg), (UK) long ton (2240 lb = 1016.05 kg), or (US) metric ton (2204.62 lb = 1000 kg), where lb is avoirdupois (standard) pound. To avoid confusion with t (tonne) and Mt (million tonnes), capitalize the abbreviations ST (= ton), LT, MT (= t), DMT (dry metric ton). Avoid adding SI prefixes, but kton (thousand short tons) and Mton (million short tons) are common. [1.153] Imperial system1.153 Imperial systemThe recommended practice is to define symbols for the traditional English units and to apply the SI rules to their use. This is done for both consistency and clarity. It avoids a number of outstanding problems caused by the traditional abbreviations for the English units. For example, the abbreviation psi is commonly used for pounds per square inch, however the correct unit of measure is pounds force per square inch, which has the algebraic symbol lbf/in.2. In this case, using the more recognizable symbol psi, defined in the text as pounds force per square inch (lbf/in.2), avoids any confusion. A more general example is the symbol for pound (lb).In general, do not add an SI prefix to an imperial unit (see Non-standard units for exceptions). The SI symbol formatting rules otherwise apply:
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