Yati and What a Letter Can Preserve

Yat is the clearest case in the Slavonic alphabet of a letter outliving the sound it was made for and remaining useful anyway. Once the vowel had merged with something else in a given language, the letter had nothing left to distinguish, and a scribe writing that language had to learn by rote which words took it. Spelling reforms in several Slavic countries eventually abolished it, at different dates and after arguments that were as much about national identity as about orthography.

But because the letter went on being written after it had stopped being heard, it preserved information the spoken language had lost. A word spelled with yat in a medieval manuscript can be traced back to a distinct vowel in the parent language even in a region where that vowel had disappeared centuries before, and comparing the same word across languages that resolved the vowel differently is how the original sound is approached at all.

That is the general lesson the letter carries. An alphabet designed for one moment of a language becomes, once the language moves on, a record of the moment it was designed for. The Slavonic scripts were fixed in the ninth century for a South Slavic dialect, and every later Slavic orthography that descends from them carries something of that dialect in its spelling.