 Morphological alternation is subject to three types of conditioning. Lexical conditioning, morphological conditioning, and phonological conditioning. Of the three, phonological conditioning is the most general and productive type of conditioning. In phonological conditioning, the sound structure determines the shape of a particular allomorph. An example of phonological conditioning in present-day English is the formation of regular plural nouns, where one of these three morphs is attached to the singular form. The suffixes are phonologically different and their distribution is complementary. The choice of allomorph can be predicted on the basis of the final stem consonant. If the base form ends with an alveolar or post-alveolar fricative over an affricate, we add the syllabic allomorph is, for example in clauses and bridges. If the preceding sound is voiced, the voiced z is selected, like in toes or dolls. If the final stem consonant is voiceless, we add the voiceless allomorph s as in cats or maps. These conditions under which a particular allomorph is selected can be generalized and transformed into morphological rules. Other examples of phonological conditioning in present-day English concern the alternation of allomorphs of the genitive s, the third-person singular s, and past tense ed. Derivational operations can also be subject to phonological conditioning, such as the choice of ion or asian, and even prefixes such as in or im.