Ablaut and umlaut

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Ablaut and umlaut are two different phonological mutations, and often refer to vowel changes under inflection.

the umlout, as in the diacritic, is not very related. See diaresis, trema, umlaut.

Ablaut is generally unconditioned, meaning it happens, but does not have a clear phonological condition, or meaning.

For example, various strong verbs in english have alternative forms, like sing, sang and sung; there is no directly obvious reason why they are the forms, and there is no single such pattern among strong verbs.

Umlaut is conditioned - it happens in specific contexts and not in others, meaning it comes from specific rules and is meaningful when interpreting a word.

I suspect the distinction is somewhat gradual.