Suppose you were reading something–either out loud or to yourself– in which a perfectly familiar word appeared, but one which you pronounced differently–probably, without hesitation–because in context it was clearly a verb rather than an adjective. That’s what would happen in sentences like “It is impossible for any creature to adequate God in his eternity” or “The aptest terms to commensurate the longitude are hard to determine.” (NB: these verbs may strike one as obsolete, but they are attested in The Century Dictionary and Cyclopedia ; not so The American Heritage Dictionary of the English Language [4th ed., 2006].) In both cases, the final syllable –ate would be pronounced with a diphthong [εi] and secondary stress; and this final vowel would differ from that of the adjective–which is pronounced with a [ə] (schwa) in the final syllable and no secondary stress. This particular vowel alternation is completely regular in all verb/adjective pairs ending in –ate, such as delegate, advocate, etc. What is interesting, however, is the distribution of the alternating vowels: diphthong in the verb, schwa in the adjective.
Again, as with the distribution of stress in verb/noun pairs, the explanation has to do with the alignment or coordination of sounds with meanings. But this time there is a twist. The alignment is not replicative as it was with verb/noun pairs–the marked stress aligning itself with the marked category (verbs), the unmarked stress with the unmarked category (nouns). Instead, it is complementary, which means that oppositely valued vowels go with oppositely valued categories. Using the same terminology as before, namely the markedness values of the entities in question, we note that the unmarked vowel–here the diphthong, which is unmarked for the phonological category of tenseness (protensity)–appears in the marked category, viz. the verb, whereas the schwa–which is marked for the category of tenseness as a lax vowel–appears in the unmarked category, viz. the adjective.
Mutatis mutandis, the same phenomenon–i.e. complementary alignment–is to be observed with adjective/noun pairs such as depraved/depravity, sane/sanity, malign/malignity, divine/divinity, where the unmarked (tense) diphthong in the stressed syllable–[ɛi] or [ai]–of the adjective alternates with the marked (lax) vowel–[æ] or [ɨ]– of the noun, since nouns are unmarked and adjectives marked within the category of nominals. It is the alignment of sound and meaning that explains the vowel alternation.