The phenomenon refers to the vowels of a word to be 'slid' to another vowel, usually e, drawn because of a following i (usually near the end of the word). This started to occur after the stage called "Old Sindarin".
Umlaut has a grammatical significance in Sindarin since it is how plural of nouns is formed.
The following verbs show how the vowels of the word-stems slid into e because of the affection of i.
- gonod "count" + -ia > genedia "to reckon"
- aglar "glory" + -ia > egleria "to praise"
- said "private" + -ia > seidia- "to set aside"
- See also: Sindarin#Pluralization
Then, all final vowels were apocoped in Sindarin, including the plural markers. All traced of plural were simply the affected vowels.
To summarize this, the envisioned history of word adar pl. edair would be like this:
- OS *atari "fathers" > *edeiri > *edeir > edair
The phenomenon is common in German and in Welsh, on which Sindarin is based on. For example the plural of W. bard (< ancient *bardos) is beird (< *bardoi)
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