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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. In Unicode, it has the codepoint U+00B5 (µ), distinct from the codepoint U+03BC (μ) of the Greek letter, so that machines can recognize it as the SI prefix symbol rather than as a letter. Most fonts use the same glyph for the two characters.
- The size of the influenza virus is about .8 to 1.2 micrometers.
- Typical bacteria are 1 to 10 micrometers in diameter. Eukaryotic cells are typically 10 to 100 micrometers in diameter. 
Symbol encoding in character sets
For historical reasons, there are two different characters in Unicode, which appear slightly different in some fonts. The micro sign (µ) is encoded in the "Latin-1 Supplement" range identical to ISO/IEC 8859-1 (since 1985), at
U+00B5 (Unicode 1.0, 1991). The Greek letter (μ) is encoded in the Greek range at
U+03BC. In circumstances in which only the Latin alphabet is available, the prefix can (unofficially) be represented using the letter u as in um for µm, or uF for µF.
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) (8th ed.), ISBN 92-822-2213-6
- Biology by Campbell & Reece tenth edition page 98 Ch. 6 A Tour of the Cell
- ISO 2955, Table 2.
- Burtis, Carl A.; Ashwood, Edward R.; Bruns, David E. (2012), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics (5th ed.).