When a sound changes because of the influence of a neighboring sound, the change is called a conditioned sound change. We have already considered a good example of a conditioned sound change from the history of English namely, the palatalization【語】顎音化of [k] before the front vowel [ī]. Notice that the only voiceless【語】清音的, 無聲的velar【語】軟顎音的 stops【語】塞音that were palatalized were those occurring before the vowel [ī]; all other velar stops remain剩下, 餘留nonpalatal. Evidence o this is Old English ku [kū], corresponding符合的; 一致的; 相同的, 對應的; 相當的to Modern English cow [ka]. It is important to focus on the phonetics語音的; 語音學的here, rather than the spelling because even though the spelling happens to have changed from k to c, this in itself does not necessarily imply暗指; 暗示; 意味著[+(that)] a change in pronunciation.
In the following discussion of sound changes, conditioned sound changes are categorized使列入...的範疇; 將...分類according to what type of conditioning條件作用, 條件反射作用is involved有關的, 牽扯在內的, 複雜的.
Conditioned Sound Change 1. Assimilation. Assimilation refers to a situation in which one sound becomes more like another sound. In Old English voiceless fricatives became voiced when they occurred between voiced sounds, e.g., the Old English word for ‘wolves’, wulfas [wulfas] came to be pronounced [wulvas]. 2. Dissimilation. Dissimilation refers to a situation in which two similar sounds become less like one another. The English word fifth [fifθ], which ends with two consecutive voiceless fricatives [f] and [θ], has undergone a dissimilating sound change in some varieties whereby the second fricative has been replaced by a voiceless stop [t], giving the pronunciation [fɪft].
Note: It is interesting (and important) to observe that in varieties where this change occurred, we talk about a diachronic sound change (θ > t / f_#), but if we compare a changed variety to a variety of English which has not undergone this change, we can see synchronic variation within