Ablaut Versus Umlaut
In Indo-European linguistics, umlaut is the vowel alternation that produces such related words as foot and feet or strong and strength. The difference in the vowels results from the influence (in Proto-Germanic or a later Germanic language) of an i or y (which has since been lost) on the vowel which (in these examples) becomes e.
To cite another example of umlaut, some English weak verbs show umlaut in the present tense.
vowel alternation bring brought (i-ou)
A-mutation and U-mutation are processes analogous to umlaut but involving the influence of an a (or other non-high vowel) or u respectively instead of an i.
In Indo-European historical linguistics the terms ablaut and umlaut refer to different phenomena and are not interchangeable. Ablaut is a process that dates back to Proto-Indo-European times, occurs in all Indo-European languages, and refers to (phonologically) unpredictable vowel alternations of a specific nature. From an Indo-European perspective, it typically appears as a variation between o, e, and no vowel, although various sound changes result in different vowel alternations appearing in different daughter languages. Umlaut, meanwhile, is a process that is particular to the Germanic languages and refers to a variation between back vowels and front vowels that was originally phonologically predictable, and was caused by the presence of an /i/ or /j/ in the syllable following the modified vowel.
From a diachronic (historical) perspective, the distinction between ablaut and umlaut is very important, particularly in the Germanic languages, as it indicates where and how a specific vowel alternation originates. It is also important when taking a synchronic (descriptive) perspective on old Germanic languages such as Old English, as umlaut was still a very regular and productive process at the time. When taking a synchronic perspective on modern languages, however, both processes appear very similar. For example, the alternations seen in sing/sang/sung and foot/feet both appear to be morphologically conditioned (e.g. the alternation appears in the plural or past tense, but not the singular or present tense) and phonologically unpredictable.
By analogy, descriptive linguists discussing synchronic grammars sometimes employ the terms ablaut and umlaut imprecisely, using ablaut to refer to morphological vowel alternation generally (which is unpredictable phonologically) and umlaut to refer to any type of regressive vowel harmony (which is phonologically predictable). Ambiguity can of course be avoided by using alternative terms (apophony, gradation, alternation, internal modification for ablaut, vowel harmony for umlaut) for the broader sense of the words.