Mu μ

Mu is the twelfth Greek letter, from Phoenician mem ('water'); its value is 40. Mu has no significant independent esoteric tradition; its interest is linguistic and, through isopsephy, numerical (in modern usage it also denotes the micro- prefix and the coefficient of friction). The Greek letters doubled as numerals in the Milesian (alphabetic) system, in which each letter carried a fixed value; the practice of summing a word's letters into a number, called isopsephy, is the Greek counterpart of Hebrew gematria and appears in inscriptions, the Greek Magical Papyri and early Christian texts. Mu's Phoenician ancestry places it in a borrowing whose exact point of contact, somewhere among Al Mina, Cyprus, Crete and Euboea in the Aegean world, is still disputed, though the borrowing itself, conventionally dated to around the 8th century BCE, is agreed to be the ancestor of every European alphabet still in use, including the Latin one covered elsewhere in this atlas.

The Greeks' own contribution was to give vowels dedicated letters of their own, something the consonant-only Phoenician script never did. Its isopsephy value of 40 belongs to the Milesian alphabetic numeral system, which also needed three further, otherwise obsolete letters, digamma (also called stigma), koppa and sampi, to cover every value from 1 up to 900; none of the three survives among the 24 letters in ordinary use today.

Mu's modern technical roles cluster almost entirely around smallness and measurement, an association with no connection to the letter's ancient Semitic root mem, meaning water, covered elsewhere in this atlas. The International System of Units uses lowercase mu as the prefix symbol for micro, one millionth, so that a micrometre or a microsecond is written with a mu in scientific notation; statisticians use mu for the mean of a population, distinguishing it from the sample mean written with a Latin letter; and physicists use mu for the coefficient of friction between two surfaces and for magnetic permeability. Particle physicists named the muon, a heavier relative of the electron discovered in cosmic ray studies in 1936, directly after the letter mu, continuing the practice of naming subatomic particles after Greek letters that also produced the later tau lepton, covered elsewhere in this atlas.

None of these 20th-century scientific conventions draws on mu's ancient sound value or its place among the Sefer Yetzirah's mother letters in the Hebrew alphabet it ultimately descends from; the modern uses are a separate, purely notational layer built on the letter's familiar shape. In the Milesian numeral system, discussed elsewhere in this atlas, mu carries the value forty, likewise unconnected to any of its later scientific senses.