Iotated Big Yus and the Completeness of the Design

Taken together, the four nasal signs at the end of the Slavonic order, the front and back nasal vowels and each of them preceded by a glide, are the best evidence in the alphabet that its designer worked systematically rather than piecemeal. The language had a set of sounds; the alphabet has a matching set of signs; and the matching is exact, including the rarest member of the set, which a designer working by ear and stopping when the common cases were covered would have missed.

That systematic quality is one of the reasons the Slavonic alphabets are held up as an unusually good fit between a script and a language. The comparison usually drawn is with alphabets that were adapted rather than designed, which typically leave some sounds unwritten and some letters idle, and both Slavonic scripts have very few of either.

It also bears on the question of how the alphabet was made. A script assembled gradually by scribes tends to accumulate exceptions; a script designed at one time by one person tends to be regular and to have gaps only where the designer's ear failed. The Slavonic evidence looks like the second, which is consistent with what the early sources say about its origin, and it is one of the few arguments about the alphabet that does not depend on the shapes of the letters.