Mu (upper case Μ, lower case μ) is the 12th letter of the Greek alphabet. In the system of Greek numerals it has a value of 40.
Mu was derived from the Egyptian hieroglyphic symbol for water which had been simplified by the Phoenicians and named after their word for water, mem. The sound "M" is still connected with the sea in many languages, as in Hebrew marah, Latin mare, German Meer, and French mer.
The letter mu appears in conjunction with alpha and omega to signify the "beginning, middle (meson) and end," a phrase first found in an Orphic verse describing Zeus, and later adopted to describe both Jehovah and Jesus.
In Aeschylus' Eumenides, the repeated moaning of the letter mu is the sound made by the sleeping Furies as the ghost of Clytemnestra begins to invoke them. It again appears as an ominous mantra in a 10th century Coptic papyrus, containing a Christian curse against perjurers that invokes the angel Temeluchos:
I adjure you by the seven perfect letters, ΜΜΜΜΜΜΜ. You must appear to him, you must appear to him. I adjure you by the seven angels around the throne of the father.
The lower-case letter mu is used as a special symbol in many academic fields. The upper case Mu isn't generally used in this way because it is normally indistinguishable from the latin M.
In classical physics and engineering:
In particle physics:
Rarely, the letter μ may be used to distinguish one item represented by "m" from another (but a better option might be to use "m", "m", "m", etc.).
Letters that arose from the Greek Μ include the Latin M and Cyrillic М.
In unicode the upper and lower case Mu are encoded at U+039C and U+3BC respectively. In ISO 8859-7 they are encoded at CCHEX and ECHEX. The micro sign is considered a distinct character by unicode for historical reasons and is found at U+00B5 as well as position B5HEX in ISO 8859-1, 3, 8, 9, 13 and 15. ISO-8859-5 also has a character that looks somewhat like lower case Mu at E6HEX but this is actually supposed to be the "cyrillic small letter tse".
Moralia, by Plutarch
Ancient Christian Magic, by M. Meyer and R. Smith (ed.), Princeton University Press, ISBN 0691004587
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