|Abstract:||This paper presents data from Tianjin tritonal sandhi patterns that challenge both traditional derivational approaches and standard Optimality Theoretic (OT) approaches to phonological alternation. If construed derivationally, Tianjin tritonal sandhi requires derivational reversals; but if construed within OT, involves combinations of opacity and transparency. The account proposed here appeals to a percolative model where phonological information from terminal nodes finds correspondences in higher nodes, such that the correspondences may be imperfect when triggered by markedness requirements. While this requires a total re-conceptualization of phonological representations, this paper argues that it is well worth it because it predicts firstly, that directionality is a derivate from branching; secondly that the depth of derivational opacity is confined by structural depth; thirdly, that constituency, not adjacency, provides the environment for triggering alternation and alternation rules can therefore be blocked when marked collocations belong to different constituencies; and fourthly that underlying entities can have split surface correspondences. All these predictions are borne out.