An alternation in the significans of occurrences of a significative unit is a kind of variation. It is therefore the task of the analyst to identify the conditions of the alternation. In the spirit of simplicity, the most general conditioning is sought that is sufficient to account for the facts. For alternations in the significans of a significative unit, this means looking first for phonological conditions. Only if these fail does one look for morphological conditions.

The alternation of the final segment in Lexemes like German Lied, nominative /liːt/, but genitive /liːdəs/, follows a very general phonological rule that says that obstruents in the syllable coda are voiceless. In order to account for the facts, it suffices to posit a morphophonemic representation {liːd}, a general rule of syllabication and this phonological rule. This is the most general description that accounts for the alternation.

The alternation observable in the significans of the noun Eiche ‘oak’ as it appears in Eichbaum ‘oaktree’ and Eichentisch ‘oak table’ as opposed to the sole noun Eiche will be approached by the same method. However, the search for a phonological condition is soon frustrated. It suffices to recall the semantically and phonologically similar noun Linde ‘linden’, which produces Lindenbaum ‘linden tree’. There are no phonological rules that apocopate or append a final /ə/ or /ən/. Instead, there is a class of nouns which have one stem alternant, ending in /ə/, which appears when they constitute a noun by themselves, and two variants, one without the /ə/ and the other ending in /ən/, which appear in compounds. The conditions of choice betweent he latter two are neither phonological nor morphological, but lexical. This is, then, the least general condition for an alternation, which the analyst accepts only in the last resort.

Since it is good methodology to first look for phonological conditions, then failing this, for morphological conditions and, faioling this, for lexical conditions, an analyst runs the risk of postulating a phonological condition at all costs.