Small Yus and What the Nasal Vowels Preserve

The nasal vowels are the strongest argument that the Slavonic alphabets were designed by someone who listened carefully to the language. The sounds are not obvious to a speaker of Greek, they had no Greek letters and no Greek parallel, and a designer working carelessly would have written them with a vowel followed by a nasal consonant and lost the distinction. Instead both alphabets give them signs of their own, and the manuscripts use those signs consistently enough that the distribution can be studied.

That consistency is what makes the letters usable as evidence. Because Polish retained the nasal vowels and the other Slavic languages did not, a word written with a yus in a ninth or tenth century South Slavic text can be matched against a Polish word a thousand years later and against the denasalised forms everywhere else, and the whole set of correspondences falls out. Reconstructing the parent language depends on such chains.

The letters also date manuscripts. As the nasal vowels were lost in the South Slavic area the scribes began to confuse the yuses with the plain vowels they had merged into, and the pattern of confusion in a text is one of the standard indicators of when and where it was copied. A manuscript that keeps them apart is either early or conservative, and telling which is its own problem.