Yeru and the Fall of the Jers

The reduced vowels are where an early Slavonic manuscript stops being a text and becomes evidence. Because they vanished in some positions and became full vowels in others, and because that process worked through the Slavic world unevenly over a long period, any scribe copying an older book had to decide what to do with letters that no longer answered to anything he said.

Three habits are visible in the manuscripts and each tells a different story. A scribe may copy them faithfully wherever his exemplar has them, which preserves the older orthography and tells you about his training rather than his speech. He may drop them where they had gone silent, which tells you the change had run in his own language. Or he may write them in the wrong places, which tells you they had become a spelling convention with no sound behind them at all, learned as a rule and applied imperfectly.

Sorting a corpus by those habits is one of the standard methods of Slavic philology, and it is how texts with no date and no colophon are placed. It works because the letters were written long after they had stopped being pronounced, which is a general truth about alphabets: a script preserves the language of the moment it was fixed, and everything after that is a negotiation between the writing and the speech.