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.
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↑ 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 prefixmicro- 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 um for μm, or uF for μF. 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 (or 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 do not), and in physical sciences academia, "μg" remains the sole official abbreviation.
In medical data exchange according to the Health Level 7 (HL7) standard, the μ can be replaced by u as well.