In the previous section we saw how the pronunciation of speech sounds can vary according to their position in the word, and that the variation is usually quite regular and can be stated in the form of "rules" which predict the variants or allophones, that will occur in each position. In this section we shall look at variation of a different kind, involving not only interchange between sounds, but also between related phonemes. We shall now touch upon the sound variations in words, their derivatives and grammatical forms of words. These variations are known as sound alternations. It is perfectly obvious that sound alternations are caused by assimilation, accommodation and reduction in speech. Alternations of consonants are mainly due to contextual assimilations: the dark  in spell alternates with the clear  in spelling. Vowel alternations are the result of the reduction in unstressed positions: combine ['kombam] (n) — combine [kam'bam] (v) where [o] in the stressed syllable of the noun alternates with the neutral sound in the unstressed syllable of the verb. In Russian alternation may be illustrated by the following pair of words: роса [рлса] — росы [росы]. The [л] sound in the unstressed syllable of the word роса alternates wilh the [o] sound in the stressed syllable of the plural form росы. То approach the matter from the phonological viewpoint it is important to differentiate phonemic and allophonic alternations. Some sound alternations are traced to the phonetic changes in earlier periods of the language development and are known as historical.
In this section we are concerned with typical, well-documented sound changes. A thousand years ago, the English spoke a language which was the direct ancestor of the English we speak today. Yet this language which we call Old English, or Anglo-Saxon, is not intelligible to present-day speakers in the written forms in which it has been handed down to us, nor would it be intelligible, if it were spoken: it has to be learned as if it were a foreign language. There are many reasons for this: there have been changes in the vocabulary, and changes in the grammar, but the primary reason for the differences between Old and Modern English is the sound changes which have taken place over the intervening period. Sound changes occur gradually; the time scale for significant changes is usually measured in hundreds of years, rather than decades. The English spoken in the 1930s sounds old-fashioned, but the changes are in details only. For two stages of "the same" language to resemble different languages require a much longer period. English speakers need assistance in reading Chaucer (fourteenth century), just as Greeks today study the Classical Greek of the fifth century BC as if it were a foreign language. Historical alternations distinguish grammatical forms of words and lexical units in the process of word-building, e.g. provide — provision (provide was at one time pronounced with [t]). The [t] was converted to [ai] by complex rules of diphthongization and vowel shift. These changes, it should he noted, reflect actual changes in the history of English. Here is another example: the OE [k] phoneme split into two separate phonemes, modem [k] and [tfl. Thus alongside cat, cool, from the OE catt, col, we have choose, chin from OE ceosan, cinn. Minimal pairs such as tin — chin; care — chair establish that Modern English has the two sounds as separate phonemes.
The alternations exemplified above are quite regular. Historical alternations mark both vowels and consonants, though the alternating sounds are not affected by the phonetic position or context, neither are they subjected to stylistic modifications. To sum up, the sound chanqes which occurred in the process of historical deveiopment of the English language are reflected in present-day English as alternations of phonemes differentiating words, their derivatives and grammatical forms. We are going to introduce here phonetic realizations of the most common historical alternations and their functions in word building and word formation. The alternations are often supported by suffixation. The following list of examples presents the most common types of alternations.