When
morphemes combine, they influence each other's sound structure (whether analyzed at a phonetic or
phonemic level), resulting in different variant pronunciations for the same morpheme. Morphophonology attempts to analyze these processes. A language's morphophonological structure is generally described with a series of rules that, ideally, can predict every morphophonological
alternation that takes place in the language. An example of a morphophonological alternation in English is provided by the
plural morpheme, written as "-s" or "-es". Its pronunciation varies among , , and , as in
cats,
dogs, and
horses respectively. A purely phonological analysis would most likely assign to these three endings the phonemic representations , , . On a morphophonological level, however, they may all be considered to be forms of the underlying object , which is a
morphophoneme realized as one of the phonemic forms {{IPA|{s, z, ɪz}}}. The different forms it takes are dependent on the segment at the end of the morpheme to which it attaches: the dependencies are described by morphophonological rules. (The behaviour of the English past tense ending "-ed" is similar: it can be pronounced , or , as in
hoped,
bobbed and
added.) The plural suffix "-s" can also influence the form taken by the preceding morpheme, as in the case of the words
leaf and
knife, which end with in the singular but have in the plural (
leaves,
knives). On a morphophonological level, the morphemes may be analyzed as ending in a morphophoneme , which becomes
voiced when a voiced consonant (in this case the of the plural ending) is attached to it. The rule may be written symbolically as → [αvoice] / [αvoice]. This expression is called Alpha Notation in which α can be + (positive value) or − (negative value). Common conventions to indicate a morphophonemic rather than phonemic representation include double slashes (⫽ ⫽) (as above, implying that the transcription is 'more phonemic than simply phonemic'). This is the only convention consistent with the IPA. Other conventions include pipes (| |), double pipes (‖ ‖) and braces ({ }). Braces, from a convention in
set theory, tend to be used when the phonemes are all listed, as in {{IPA|{s, z, ɪz}}} and {{IPA|{t, d, ɪd}}} for the English plural and past-tense morphemes and above. For instance, the English word
cats may be transcribed phonetically as , phonemically as and morphophonemically as , if the plural is argued to be underlyingly , assimilating to after a voiceless nonsibilant. The tilde ~ may indicate morphological alternation, as in or {{IPA|{n iː~ɛ l}, {n iː~ɛ l+t}}} for
kneel~knelt (the plus sign '+' indicates a morpheme boundary). ==Types of changes==