In IE languages there is a special kind of vowel alternation. It is called gradation. This is found, for example, in Russian in such pairs as áðåñòè – áðîä, áåðó – âûáîð – áðàòü.
The term ablaut (from German ab- in the sense "down, reducing" + Laut "sound") was coined in the early 19th century by the linguist Jacob Grimm. However, the phenomenon itself was first observed more than 2,000 years earlier by the Sanskrit Grammarians.
As to the origin of gradation, some linguists believe that the three variants of a root, distinguished by gradation, are due to stress conditions:
full stress brings about the high degree – o,
weakened stress – the medium degree – e,
and unstressed position – Æ (zero).
In Germanic languages gradation can be found both in word-formation and form-building.
Word-formation:
OE þencan (äóìàòü)
- þanc (ìûñëü):
ĕ – ă
Goth. drigkan (ïèòü)
- dragk (ïèòü¸):
ĭ – ă (< ĕ – ă)
The system of gradation in Germanic languages is best seen in the so-called strong verbs. In Gothic, the system of gradation is very clear, whereas in other Germanic languages, including English, it has undergone such changes that its original laws are hard to discover.
Six different ablautseries exist in the Germanic languages. These are referred to as the six classes in which the strong verbs can be subdivided. It should be noted that the first 3 classes actually show an identical ablaut and root shape, e.g.: Gothic – ĭ – ă – Æ + R, where R is a resonant (i in Class 1, u in Class 2, and n in Class 3). Classes 4 and 5 don’t reveal the zero grade positions. Thus Classes 1-5 contain qualitative ablaut, that is, the alternation of different vowels.
Class 6 is considered to be a typical exampe of quantitative ablaut, though it presents the interchange of short ăand long ō. That fact makes Germanic quantitative ablaut a poorly understood phenomenon [Wiese]. But since Germanic *ă partly goes back to IE shortŏ, the quantitative pattern here is easily discernible, even though Germanic has transformed the purely quantitative alternation into a quantitative *and* qualitative one.
The 7th class contains reduplication, that is, addition of an extra syllable consisting of the initial consonant and the vowel [e] (spelt ai) in the past tense, sg/pl.,e.g.: slēpan – saislēp (to sleep), fahan – faifah (to catch). Some verbs of this class contain ablaut, e.g.: letan – lailōt (to let), tekan – taitōt (to touch).
Reduplication can be found in many IE languages (Latin, Greek, Sanskrit). The origin of reduplication was probably emphatic repetition meant to stress the completion of the action (IE perfect forms).