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The symbol for the prefix comes from the Greek letter μ (mu). It is the only SI prefix which uses a character not from the Latin alphabet. "mc" is commonly used as a prefix when the character "µ" is not available; for example, "mcg" commonly denotes a microgram. Also the letter u instead of µ is allowed by one of the ISO documents.
- Typical bacteria are 1 to 10 micrometres in diameter.
- Eukaryotic cells are typically 10 to 100 micrometres in diameter. 
- Prefixes adopted before 1960 already existed before SI. 1873 was the introduction of the CGS system.
Symbol encoding in character sets
The official symbol for the SI prefix micro- is a Greek lowercase mu (μ). For reasons stemming from its design, there are two different characters in Unicode, which appear slightly different in some fonts, although most fonts use the same glyph. The micro sign (µ) is encoded in the "Latin-1 Supplement" range identical to ISO/IEC 8859-1 (since 1987), at
U+00B5 (Alt+0181), residing at this code point also in DEC MCS (since 1983) and ECMA-94 (since 1985). The Greek letter (μ) is encoded in the Greek range at
U+03BC (Alt+956). According to The Unicode Consortium, the Greek letter character is preferred, but implementations must recognize the micro sign as well.
In circumstances in which only the Latin alphabet is available, ISO 2955 (1974, 1983), DIN 66030 (Vornorm 1973; 1980, 2002) and BS 6430 (1983) allow the prefix
μ to be substituted by the letter
u (or even
U, if lowercase letters are not available), as, for example, in
Similar, capacitor values according to the RKM code defined in IEC 60062 (IEC 62) (since 1952), EN 60062, DIN 40825 (1973), BS 1852 (1974), IS 8186 (1976) etc. can be written as
4U7) instead of
4µ7 if the Greek letter
µ is not available.
Other abbreviating conventions
In some health care institutions, house rules deprecate the standard symbol for microgram, "μg", in prescribing or chart recording, because of the risk of misdose via the misreading of poor handwriting. The two alternatives are to abbreviate as "mcg" or to write out "microgram" in full (see also List of abbreviations used in medical prescriptions). But this deprecation, focused on bedside misdose avoidance in contexts where handwriting is often present, does not extend to all health-care contexts and institutions (for example, some clinical laboratories' reports adhere to it, whereas others don't), and in physical sciences academia, "μg" remains the sole official abbreviation.
- International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), ISBN 92-822-2213-6, archived (PDF) from the original on 2017-08-14
- ISO 2955, table 2.
- Biology by Campbell & Reece tenth edition page 98 Ch. 6 A Tour of the Cell
- Prefixes of the International System of Units, International Bureau of Weights and Measures (page visited on 9 May 2016).
- (Unicode 1.0, 1991)
- Unicode Technical Report #25
- ISO 2955-1974: lnformation processing - Representations of SI and other units for use in systems with limited character sets (1st ed.). 1974.
- "Table 2". ISO 2955-1983: lnformation processing - Representations of SI and other units for use in systems with limited character sets (PDF) (2nd ed.). 1983-05-15. Retrieved 2016-12-14. 
- Vornorm DIN 66030 [Preliminary standard DIN 66030] (in German). January 1973.
- DIN 66030: Darstellungen von Einheitennamen in Systemen mit beschränktem Schriftzeichenvorrat (in German) (1st ed.). 1980.
- "Neue Normen für die Informationsverarbeitung". Computerwoche (in German). 1981-01-09. Archived from the original on 2016-12-14. Retrieved 2016-12-14.
- DIN 66030:2002-05 - Informationstechnik - Darstellung von Einheitennamen in Systemen mit beschränktem Schriftzeichenvorrat [Information technology - Representation of SI and other units in systems with limited character sets] (in German). Beuth Verlag. May 2002. Retrieved 2016-12-14.
- Burtis, Carl A.; Ashwood, Edward R.; Bruns, David E. (2012), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics (5th ed.).
- "Commonly Used UCUM Codes for Healthcare Units". HL7 Deutschland e.V. Retrieved 2015-11-21.