Psi stands forty first, writes the cluster ps, and carries the numeral 700 from Greek psi.
Like ksi it writes nothing Slavonic needs. The cluster can be spelled with the letters for p and s, and in native vocabulary it is. The letter exists to preserve the Greek spelling of Greek words and to occupy its place in the Greek numeral series.
One category of word kept it conspicuously in use. The psalms, the psalter and the vocabulary built around them all begin with this cluster in Greek, and the psalter was the single most copied, most read and most memorised book in the Slavonic tradition, used for teaching literacy as well as for prayer. A letter that appears in the title of the book everyone learned to read from is not obscure, whatever the rest of the vocabulary looks like.
Its numeral value of 700 put it in every date in Slavonic books during long stretches of the reckoning from the creation of the world.
It was among the letters removed from secular printing with the introduction of the civil type in the early eighteenth century, the subject of the companion article, and it remains in the church books today.
The letter also does a job no rule required it to do. Because Slavonic writes the cluster with this single sign in words taken from Greek and with two separate letters in native words, the choice of spelling tells a reader where a word came from. The same holds for ksi. A scribe following the convention was therefore marking the vocabulary of the church off from the vocabulary of the language, silently and on every page, and a modern reader can use the distinction the same way, as a first indication of whether a word is a borrowing before looking anything up.