This chapter and the next deal with rules that account for allomorphy, which can affect both the spelling and the pronunciation of a morpheme. The processes that can change the shape of a morpheme are covered by three types of rules:

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

2. deletion rules – delete a sound, or sounds, from a morpheme;

3. 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 a 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.