In contrast to GB where the completed morpho-syntactic derivation was merely dumped into PF and (LF) with a "good bye and don't come back", phase theory establishes a two-way pipe between the morpho-syntactic and the phonological (and semantic) module. Actors on both ends are not free anymore to do what they want: their theories and analyses may make predictions on the other end. Intermodular argumentation in a phase-based architecture can thus build on the logical fact that interpretational units—the pieces which are designated for spell-out—must be the same in morpho-syntax and in phonology. Intermodular argumentation probably provides stronger evidence than what can be produced by modular-internal reasoning: it offers the maximal degree of independent assessment that linguists can expect without leaving their discipline. The intermodular argument that is made in this paper concerns the phase edge: in current syntactic phase theory, only the complement of (the head of) the phase head is actually interpreted. In other words, the sister of X° is spelled out. Based on phonological evidence of cyclic spell-out, a number of theories have been proposed which have variable takes on how spell-out works. Among these, only one—Kaye (1995)—spells out the sister of interpretation-triggering affixes. The point that is made, then, is that only this phonological theory is able to offer a unified spell-out mechanism for syntactic and phonological effects of the same thing: cyclic derivation. This is how syntax can referee the competition of phonological theories. Finally, an issue is discussed that appears to stand in the way of a uniform spell-out mechanism: in syntax phasehood is a property of node labels (node-driven phase), while in phonology node labels are irrelevant: nodes inherit phasehood, which is a lexical property of affixes (piece-driven phase). It is pointed out that den Dikken's (2007) Phase Extension is a step in the direction of piece-driven phase.