The Well-Formedness Condition of current multi-linear phonology includes three clauses, which we will discuss in the context of the relations between tones and vowels. In this context, the WFC states that:
1: For every vowel there is at least one tone.
2: For every tone there is at least one vowel.
3: Association lines may not cross.
These clauses are stated as though they are unrelated to each other and arbitrarily fixed; they call out for unification and explanation. They may be seen to follow trivially from features of linguistic spatio-temporal organization. Sagey (1986) gave a proof of the same conclusion using different assumptions, namely that phonological representations are continuous rather than discrete.