Yu Ю

Yu stands thirty third and writes the sequence of a glide followed by the vowel u. It carries no numeral value.

Old Church Slavonic wrote such sequences with letters of their own rather than with two signs, and yu is the member of that group that survived everywhere. It is present in every modern Cyrillic alphabet, unchanged in shape and in value, where its companions in the iotified series have mostly gone.

The reason it survived and they did not is that the sequences the others wrote either disappeared or were absorbed. The iotified e fell together with the plain e in most of the East Slavic area. The iotified a became the modern letter for ya. The iotified yus letters lost their nasal vowels and had nothing left to write. Yu had no such fate: the sequence it represents remained common and distinct, so the letter kept its job.

In the modern alphabets it does a second job as well. Because Cyrillic marks consonant softness on the following vowel, this letter after a consonant no longer writes a glide at all; it writes a plain u and says that the consonant before it is soft. The same letter therefore means one thing at the start of a word and another in the middle of one, which is a general property of the Cyrillic vowel letters and the point of the companion article.

A glide before a vowel is easy to say and awkward to write, and Slavonic writing has taken three different approaches to it, all of them still visible.

The first is a dedicated letter for the sequence. That is what the iotified series is: single signs for the glide plus a, the glide plus e, the glide plus u and the glide plus each of the two nasal vowels. Several of them are transparently ligatures of a glide sign and a vowel letter, so the method is really a shorthand for the second approach rather than a separate principle.

The second is to write the two elements separately, glide sign plus vowel letter, which the manuscripts also do, inconsistently, alongside the ligatures.

The third emerged much later and is what the modern alphabets use. The iotified letters were reinterpreted: instead of meaning glide plus vowel, they came to mean either glide plus vowel at the start of a syllable, or plain vowel with a soft consonant in front of it. One sign, two readings, determined by position. That reinterpretation is what allowed Russian to write consonant softness without adding any diacritics to the consonants.

The shift is a good example of how a writing system changes without anyone deciding to change it. Nobody redefined these letters. The sound system moved underneath them, the letters went on being written in the same places, and their meaning was re-derived by readers from where they found them.