There are various types of assimilation, all of which have in common that one sound (the target) copies a feature or features of a sound in its environment (the source). Assimilation may be classified in a number of ways.
A. By direction
- Anticipatory (Regressive): The source of the assimilation is the second sound in the sequence. An example from English: n → m in the phrase ten billion tem bɪljən. Here it is the bilabial place of articulation which has been copied from the following b.
- Perseverative (Progressive): The source of the assimilation is the first sound in the sequence. An example from English: n→ m in the word happen hæpm. Here it is the bilabial place of the preceding p which has been copied. Also called carry-over assimilation.
- Coalescent (bidirectional): two segments combine to give a single output segment. Example from English: dɪd juː → dɪdʒuː.
B. By distance
- Contact: the source and target are adjacent, though not necessarily in the same syllable or word. Examples as in A1. and A2. above
- Distant: the source and target are separated by other segments. This is most common with vowel sounds and is called vowel harmony or umlaut. Distant assimilation of consonant features does occur in child speech where it is usually called consonant harmony.
C. By feature(s) copied
- Place: The place of articulation of a sound is altered to agree with some sound in its environment. In English, for example, alveolar consonants are particularly susceptible targets for this kind of assimilation. An example is good girl gʊg gɜːl, where the plosive at the end of the first word copies the velar place of the following consonant.
- Voice: Examples can be found where voiced consonants become voiceless, or voiceless consonants become voiced, under the influence of a neighbouring segment. An example of the former change often occurs in the English phrase has to hæs tuː. An example of the latter change can be seen in French: as (ace) as, as de pique (ace of spades) az də pik.
- Manner: The manner of articulation of a sound is altered to agree with the manner of a sound in the environment. An example of this from English is the occasional copying of nasal manner, as in the phrase good night gʊn naɪt.
D. By extent
- Partial: only some phonetic features are copied from source to target.
- Complete: the target is changed to become identical with the source. An example of this is the definite article əl in Arabic. The final consonant changes to become identical with the initial consonant of a following noun, if this consonant is apical. Example: əd dar the house.
Assimilation and coarticulation are very similar phenomena. The distinction between them is largely one which rests on the analyst’s theoretical outlook. In traditional phonemic phonology, assimilation results in a phoneme different from the target, whereas coarticulation does not. Also the term assimilation is usually reserved for those changes which are completely optional. Coarticulation on the other hand is usually deemed to be more or less automatic and obligatory.