Fill Vowels

Fill Vowels are also called Mobile Vowels and Fugitive Vowels. They come and go inside Russian words as they decline or conjugate, so that отец suddenly loses its E and becomes отца, while земля as suddenly gains an E to become земель.

Why? What's going on here?

 It's all explainable - if a bit historical and arcane. And it starts with "The Fall of Jers". (And no, "Jers" isn't a city, which was my first thought on hearing the phrase!)

Until roughly a thousand years ago, the Slavic ancestor of Russian had two more vowels – written with the letters ъ and ь, ‘uh’ and ‘ih’ respectively. These very short vowels, which linguists call ‘jers’, eventually suffered the same fate as the final ‘silent e’ in English words like, well, ‘like’ – they stopped being pronounced. Some of them turned into other vowels (mostly O and E) but some simply dropped out of the word entirely. And the ones that remained ceased being vowels entirely and took up their current role as diacritics.

This process - losing these short vowels - meant that Common Slavic, which, up until then, had ended every syllable with a vowel, now had syllables ending in consonants. It also meant that the CC non-syllabic roots came into being. And that is where fill vowels come into play.

For example, take a word like сънъ. Remembering that the ъ was a vowel, this word would have been pronounced ‘suhnuh’. When jers were lost from a word, if there were more than one, the last one, bearing the least stress, was lost; the preceding one was converted to another vowel. So, сънъ became сoн. But in other cases, there was a ‘real’ vowel at the end – genitive сънa – and so it was the other that was lost: сна.

Similarly дьнь became день* and дьня became дня. While in feminine and neuter nouns – with their strong vowel endings – it’s the genitive plural that changes. For instance, сестъра becomes сестра, while сестър became сестер.

The most common fill vowel is O: Examples: ОКНО – ОКОН; ШУТКА – ШУТОК; ОГОНЬ – ОГНЯ Sometimes the O is a Ё: Examples: ЛЁД – ЛДА; СЕСТРА - СЕСТ ЁР; ПА ЁК – ПАЙКА

The next most common fill vowel is a genuine E: Examples: ЧЕЙ – ЧЬЯ; ДЕНЬ – ДНЯ; ЗЕМЛЯ – ЗЕМЕЛЬ; БЕЙ – БЬЮ

И** is more rarely seen.

In adjectives and nouns, it is used only next to a jot. Examples: ТРЕТИЙ – ТРЕТЬЯ; СОБАЧИЙ – СОБАЧЯ; ЯЙЦА – ЯИЦ

In verbs, И is found in imperfectives formed from prefixed perfectives: Еxamples: СОБРАТЬ – СОБИРАТЬ; УМРЕТЬ – УМИРАТЬ; ПОМНИТЬ – ПОМИНАТЬ.

**Note that in six verb roots the fill vowel is Ы:
З/В: ПРИЗВАТЬ – ПРИЗЫВАТЬ
М/К: ЗАМКНУТЬ – ЗАМЫКАТЬ
Р/В: ПОРВАТЬ – ПОРЫВАТЬ
С/Л: ПОСЛАТЬ – ПОСЫЛАТЬ
С/П: ЗАСПАТЬ – ЗАСЫПАТЬ
Т/К: ЗАТКНУТЬ – ЗАТЫКАТЬ

 * Remember, the Ь here is marking the final H as soft; it's not a vowel.