Yo and the Limits of Matching the Two Alphabets

For most of their length the two Slavonic alphabets can be laid side by side letter for letter. They share an order, a set of names and a set of sounds, and a reader can move along the two lists together from the first letter to somewhere past the twentieth without difficulty. At the end of the order that stops working.

The reason is that the Slavic material at the end of the alphabet is where each script was doing its own work. Greek supplied the frame for the first two thirds; after that both alphabets had to provide signs for affricates, reduced vowels, nasal vowels and glide sequences, and they did not make identical choices about which combinations deserved a letter of their own. One script may write with a single sign what the other writes with two, and one may have a letter the other simply lacks.

So a table of correspondences between the two alphabets is complete at the start and ragged at the end, and the ragged part is the interesting part, because it is where the design decisions are visible. The atlas records the correspondences it can support and leaves the rest unmatched. An unmatched letter is a piece of information about how the two scripts differ, and filling the gap for the sake of symmetry would destroy it.