Morphophonological Rules

Morphophonogical rules combine morphological and phonolgical processes. They focus primarily on the sound processes which take place in morphemes to form words. In morphophonological rule notation double slashes, //, are used.

The underlying form of the English plural suffix is /z/. This is the morphophoneme in words such as beds, days and tables. The other two allomorphs are /s/ as in cats and /əz/  as in roses. However, in many varieties of English such as Received Pronunciation, the plural suffix of roses is in fact /Iz/. The allomorph /z/ is considered underlying on the basis of wider distribution. It follows vowels and voiced consonants with the exception of sibilants. The allomorph /s/ follows voiceless consonants with the exception of sibilants and the allomorph /əz / only follows sibilants.

The plural boots can be represented with phonetic, phonological and morphophonological representations. They are as follows:

[buts] phonetic representation
/buts/ phonemic representation
//butz// morphophonological representation

With /z/ selected as the morphophoneme of the plural suffix, two other rules are needed to derive the other forms. They are an epenthesis rule and a devoicing rule. Here we can observe the interaction of the two rules with the plurals beds, boots and wishes:

UR /bɛd+z/

epenthesis --
devoicing --
PR [bɛdz]

UR /but+z/

epenthesis --
devoicing  buts
PR [buts]

UR /wIʃ+z/

epenthesis wIʃəz
devoicing --
PR [wIʃəz]

Epenthesis only applies between two sibilants and is thus blocked in [buts]. Devoicing only applies between a voiced and a voiceless consonant and is thus blocked in [wIʃəz]. The rules of ephenthesis and devoicing must be ordered. The reason is that if devoicing were the first rule, the plural suffix would devoice. Thus the phonetic realization of [wIʃəz] would end with the suffix variant of [buts].

Plural forms such as knives and leaves are unique because they have two voiced fricatives. This is not the case in the form cats, which has two voiceless fricatives. The phonetic representation is derived from the underlying representation with progressive devoicing: the voiceless plosive of cat devoices the underlying plural suffix /z/.

In forms such as knives and leaves, though, progressive devoicing does not take place. Here the final consonant of knife and leaf becomes voiced when it is combined with the plural allomorph /z/. An additional rule can be created, stem-final fricative voicing, to account for these forms. This only applies to morphemes which end with a voiceless labiodental fricative. Here is the analysis of leaves:

UR lif+z

epenthesis --
devoicing --
stem-final fricative voicing //livz//
PR [livz]

The double slashes indicate that this is a morphophonological rule and not a categorical one. It does not apply to forms such cliffs and chiefs nor to leaf's as in The leaf's changing colour. In certain cases, stem-final fricative voicing is optional as in roof/rooves and wharfs/wharves.

The interaction of morphological and phonological processes can be captured with morphophonological rules. In English these can be illustrated with rules for plural suffixation and  stem-final fricative devoicing. Morphophonological rules are distinct from phonological rules because they apply specifically to morphemes.