Language change spreads in two ways: outwardly through a community, and inwardly through a language. Let us now consider this inner linguistic burrowing. Like a seed, any change is likely to have small beginnings. But if it puts down firm roots, it can develop into a massive growth which affects the whole landscape. This spreading process is the topic of this and the next chapter. This chapter will deal with sound change, and the following with syntactic change.

Just as a seed is likely to enter the ground where the soil is soft, such as a crack between paving stones, so a sound change is likely to creep into a language at a vulnerable point. In the words of Edward Sapir, a sound change is a ‘consummated drift that sets in at a psychologically exposed point and worms its way through a gamut of psychologically analogous forms’. We need to know what is meant by ‘a psychologically exposed point’, and how it ‘worms its way through’.

Until relatively recently, this was a question neglected by the majority of linguists, who did not realize that anything needed explaining. It was widely assumed that a sound change affected all the relevant words in a dialect simultaneously.