Not to be confused with the Greek letters Ϻ or υ, the Latin letters M or u, or the Cyrillic letter М. For the micro sign, see Micro-. For other uses, see MU.
Mu (; uppercase Μ, lowercase μ; Ancient Greek μῦ [mŷː], Greek: μι or μυ—both [mi]) is the 12th letter of the Greek alphabet, representing the voiced bilabial nasal IPA: [m]. 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, to become the letter mem (𐤌). Letters that derive from mu include the Latin M and the Cyrillic М, though the lowercase resembles a small Latin U (u).