Sou: The Coptic Numeral Sign (Value 6), from Greek Stigma

Sou is the sixth sign of the Coptic alphabet and writes no sound whatever. It exists because Coptic took over the Greek alphabetic numeral system, and that system had a problem in its sixth position. The oldest Greek alphabet contained a letter, digamma, which fell out of use as a sound in most Greek dialects long before the classical period, but its numeral value of six survived in the counting series because dropping it would have shifted every value after it. Greek scribes went on writing a sign in that place, which in the manuscript tradition merged with the ligature called stigma.

Coptic inherited the arrangement complete, so Coptic alphabet charts list sou between ei and zeta, and Coptic scribes wrote it whenever they needed the figure six. No Coptic word contains it. The sign is a clean demonstration that an alphabet and a numeral series are two systems that happen to share a set of shapes, and that the two can come apart. Greek needed two further signs of the same kind to cover ninety and nine hundred, koppa and sampi, and those too are numeral signs rather than letters, appearing in Coptic manuscripts where a number requires them but never in a Coptic word. Coptic charts vary in whether they print them in the alphabet at all, which is a matter of editorial convention rather than of dispute about the script.