Big Yus and the Loss of the Nasal Vowels

The disappearance of the nasal vowels is one of the largest changes in the history of Slavic, and the two yus letters are how it is tracked. Because they were written before the change and continued to be written for a time afterwards, the manuscripts record the process in progress rather than only its result.

The signs of that are visible in the way scribes used the letters. Early texts keep the two nasal vowels apart and keep both apart from the oral vowels they would eventually become. Later texts begin to confuse them, first in particular positions and then generally, and eventually a scribe writes a yus where the older orthography had an oral vowel, which shows the letter has become a spelling convention with no sound behind it. Each stage can be dated approximately and located approximately, and the pattern differs between regions.

That is why the yus letters are among the first things a Slavist looks at in an unplaced manuscript. They are a clock that was running while the books were being copied. The letter itself is not the change and does not cause it; it is a fixed point against which the change can be measured, which is what an old orthography always is once the language has moved past it.