Sound interchange may be defined as an opposition in which words or word forms are differentiated due to an alternation in the phonemic composition of the root. The change may affect the root vowel, as in food n : : feed v; or root consonant as in speak v : : speech n; or both, as for instance in life n : : live v. It may also be combined with affixation: strong a : : strength n; or with affixation and shift of stress as in 'democrat : : de'mocracy. The process is not active in the language at present, and oppositions like those listed above survive in the vocabulary only as remnants of previous stages. Synchronically sound interchange should not be considered as a method of word-building at all, but rather as a basis for contrasting words belonging to the same word-family and different parts of speech or different lexico-grammatical groups. The causes of sound interchange are twofold and one should learn to differentiate them from the historical point of view. Some of them are due to ablaut or vowel gradation characteristic of Indo-European languages and consisting in a change from one to another vowel accompanying a change of stress. The phenomenon is best known as a series of relations between vowels by which the stems of strong verbs are differentiated in grammar (drink — drank — drunk and the like). However, it is also of great importance in lexicology, because ablaut furnishes distinctive features for differentiating words. The examples are: abide v : : abode n; bear v : : burden n; bite v : : bit n; ride v : : road n; strike v : : stroke n. The other group of cases is due to an assimilation process conditioned by the phonemic environment. One of these is vowel mutation, otherwise called umlaut, a feature characteristic of Germanic languages, and consisting in a partial assimilation to a succeeding sound, as for example the fronting or raising of a back vowel or a low vowel caused by an [i] or [j] originally standing in the following syllable but now either altered or lost. This accounts for such oppositions as full a : : fill v; whole a : : heal v; knot n : : knit v; tale n : : tell v. The process will be clear if we follow the development of the second element in each pair. ModE fill It will be recalled in this connection that the systematic character of the language may manifest itself in the analogy between word-building processes and word inflection. It is worthy of note that not only are these processes similar, but they also develop simultaneously. Thus, if some method is no longer productive in expressing grammatical categories, we shall also observe a parallel loss of productivity in expressing lexical meaning. This is precisely the case with root inflection. Instances of root inflection in the formation of the plural of nouns (goose — geese, foot — feet, tooth—teeth) or the Past Indefinite and Participle II of verbs (sing — sang — sung, drive — drove — driven, tear — tore — torn) exist in the language as the relics of past stages; and although in the case of verbs the number of ablaut forms is still very great, no new verbs are inflected on this pattern. The same may be said about word-building by sound interchange. The type is not productive. No new words are formed in this way, yet sound interchange still stays in the language serving to distinguish one long-established word from another. Synchronically, it differentiated parts of speech, i.e. it may signal the non-identity of words belonging to different parts of speech: full a : : fill v; food n : : feed v; or to different lexico-grammatical sets within the same part of speech: fall intransitive v : : fell causative v; compare also lie : : lay, sit : : set, rise : : raise. Derivation often involves phonological changes of vowel or consonant: strong sl : : strength n; heal v : : health n; steal v : : stealth n; long a : : length n; deep a : : depth n. Major derivative alternations involving changes of vowel and /or consonant and sometimes stress shift in borrowed words are as follows: delicacy n : : delicate a; piracy n : : pirate n; democracy n : : democrat n; decency n : : decent a; vacancy n : : vacant a; creation n : : create v; edify v : : edification n; organise v : : organisation n; agnostic a : : agnosticism n. Some long vowels are retained in quality and quantity; others are shortened, and there seems to be no fixed rule, e.g. [a:] tends to be retained: artist n : : artistic а; [э:] is regularly shortened: ‘permit n : : per'mit v.