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 or in the syllable following the modified vowel.