Ksi Ѯ

Ksi stands fortieth, writes the cluster ks, and carries the numeral 60 from Greek xi.

Slavonic did not need it. The cluster it writes could be spelled perfectly well with the letters for k and s, and in ordinary Slavic vocabulary it is. The letter exists because Greek had it, because the Greek numeral series needs a sign at 60, and because a very large part of the vocabulary of the church books is Greek names and Greek technical terms whose spelling the tradition preserved.

That gives it a peculiar status. It is a letter of the alphabet in the sense that it appears in the alphabet lists, has a name and carries a number, and it is not a letter of the language in the sense that no Slavonic word requires it. A scribe could go a long way through an ordinary text without writing one.

Its numeral function was the more important of its two jobs and the one that kept it alive longest. Sixty is needed, and no other letter could supply it.

It was among the letters removed from secular printing when a civil type was introduced in the early eighteenth century, and it survives today only in the church books, where the old numeration is still in use and the Greek spellings are still preserved.

Four letters of this alphabet write nothing Slavonic requires: ksi, psi, fita and izhitsa. Omega belongs with them in function though not in sound, since it duplicates a letter Slavonic already had. Together they form a distinct class, and recognising it explains a good deal about how the alphabet was used and why it was eventually cut down.

They exist for two reasons. The first is transliteration. The church books are full of Greek names and Greek terms, and spelling them with the Greek letters preserves the identity of the word, so that a reader who knows both languages can see through the Slavonic spelling to the original. That mattered for names above all, where a variant spelling can look like a different person.

The second is arithmetic. The Greek numeral series needs twenty seven signs, and these letters carry 60, 700, 9 and 400. Removing any of them breaks the number system, and for as long as the numbers were in use the letters were not removable at any price.

Both reasons weakened together. When secular printing adopted Arabic numerals the arithmetic argument collapsed, and when secular writing stopped being dominated by translated Greek the transliteration argument weakened with it. The letters went out of the civil alphabet within a generation of the numerals going, and stayed in the church books, where both reasons still hold and where all four are still set and still read.