This chapter and the next deal with rules which account for predictable allomorphy. The rules of predictable allomorphy are of three types:

replacement rules – replace one sound by a different sound in a certain position in the morpheme;

deletion rules – account for the loss of a sound, or sounds, from a morpheme;

expansion rules – expand a morpheme by inserting a new sound within the existing structure of the morpheme.

This classification exhausts the logical possibilities of what can happen to one phonetic segment. It can be replaced, deleted, or some sequence of sounds can be expanded.

Assimilation and types of assimilation

Assimilation rules are replacement rules which have the effect of making one vowel or consonant more similar to, or even identical with, another. In principle, assimilation can affect both vowels and consonants; most instances of assimilation discussed below, however, are cases of consonantal assimilation.

The process of assimilation can be described in terms of the target, the direction, and the scope of the resulting similarity. Assimilation can target some or even all of a sound's features: voicing, place, or manner of articulation. Depending on the direction of the influence between the sounds we find right-to-left assimilation, when the influence is from the second to the first sound, i.e. A ← B, also known as regressive assimilation, and left-to-right, when the first sound influences the second, the A → B type, known as progressive assimilation.