Ksi: The Coptic Letter (Value 60), from Greek Xi

Ksi is the fifteenth letter of the Coptic alphabet and carries the numeral value sixty. It is the Greek xi, and like theta and psi it writes a cluster of two consonants rather than a single one, standing for k followed by s. Coptic treats it as an optional shorthand. The same word may appear with ksi in one manuscript and with the two letters spelled out in another, and neither is a mistake, so the letter is markedly less frequent in Coptic than most of the alphabet. That rarity gives its appearances a certain evidential value, because a scribe who consistently prefers the compact form is following a habit that can be traced across a body of work.

In the numeral series ksi is in constant use for sixty regardless of how often it appears as a sound, which is another instance of the two functions of the alphabet running on separate tracks. The whole group of cluster letters raises the same question about Coptic that sou raises about the numerals: a borrowed system arrives with parts that do not quite fit, and what a script does with those parts is as informative as what it does with the ones that do. Ksi is also a reminder that letter frequency in a script is not a property of the alphabet but of the language and the scribe together. In Greek the letter is uncommon but obligatory where it belongs; in Coptic it is uncommon and optional, so its rate varies from one manuscript to another in a way that reflects a decision rather than the vocabulary. Editors record that variation because it groups manuscripts, and a habit shared across several books is often the trace of a single scriptorium or a single teacher.