|Abstract:||In the Tianjin dialect, casual utterance of familiar trisyllabic sequences induces deletion of phonological segments, which interacts with the rich Tianjin tone sandhi system. This paper explains that Casual Speech Elision (CSE) is the result of syllable mergers triggered by mora deletion. Sonority peak requirements determine the kind segments retained and the vowel coalescence patterns found in CSE. Since tone features hang off the mora, CSE could set off derived sandhi-triggering sequences in this sandhi-rich dialect. The intricate patterns here bring to the foreground the gaps in earlier analyses of Tianjin tone sandhi, leading to an account that shows how a combination of OCP constraints and iambicity based on tonal complexity can provide a fuller account with OT devices of comparative markedness and constraint conjunction.