The surface realization of a linguistic expression can often be predicted from the form of paradigmatically related items that are not contained within it: in Latin, the nominative singular of a noun can often be inferred from the genitive; in French, the final consonant of a prenominal masculine adjective in liaison can typically be predicted from the feminine; in Romanian, the plural form of a noun determines whether its stem will exhibit palatalization before the derivational suffix /-ist/. Such instances of phonological paradigmatic dependence without containment have been claimed to challenge cyclic models of the morphosyntax-phonology interface. In this article, however, they are shown to be established indirectly through the acquisition of underlying representations. This approach correctly predicts that phonological paradigmatic dependencies are never systematically extended to new items if they involve suppletive allomorphy rather than regular alternation, whilst those surface phonological properties of derivatives that are under strict phonotactic control evade paradigmatic dependence on the inflectional forms of their bases. Theories relying on surface-to-surface computation fail to recover these empirical predictions because they are inherently nonmodular, positing generalizations that promiscuously mix phonological, morphosyntactic, and lexical information. Underlying representations, therefore, remain indispensable as a means of establishing a necessary modular demarcation between regular phonology and suppletive allomorphy.