As shown, the different changes affect the surface form in different ways. Deletion turns the surface form into no phoneme at all. Insertion inserts a phoneme where none had previously been. Fissure adds two phonemes where there had been only one (the /hɔrsɨz/ case demonstrates this). Fusion causes two separate phonemes to become a single one, perhaps adopting some properties of both; change of value, quite common and demonstrated above by /s/ becoming voiced as /z/ in our cats and dogs example, changes the phoneme into another one entirely; and perhaps less common, change of order simply involves two phonemes switching places. These are all, of course, determined by the phonetic environment surrounding them.
2. In cases of complementary distribution, phonologists seek to find a rule that explains why there is a variant. If a rule is found, then the variant which is subject to the rule is clearly the one subject to the rule, as it can only be explained by it. In complementary distribution, the two sounds are, by necessity, mutually exclusive and cannot occur in the same environment. The underlying form is the form that is not affected directly by a rule. So, for example, in German, which has final devoicing, the end forms of /p/, /t/, and /k/ for /b/, /d/, and /g/ are the variants, since they are subject to the rule in these particular situations, but are only expressed in the way that they are because they happen to be at the end of a syllable, and would be expressed in their normal forms in other environments.
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