This paper investigates tone sandhi phenomena in the Chinese dialect of Tianjin, which are noteworthy for the rule application directionalities observed in tri-tonal strings. The rule application directionalities appear to be ungoverned, as none of the principles proposed to date that may contribute to determining directionalities can account for them. Based on the constraint-based theory of OT, this paper shows that the rule operation directionalities in Tianjin are by no means ungoverned. Normally tone sandhi applies from left to right for identity reasons. This is captured by the OO-faithfulness constraint Ident-BOT, which requires identity between prosodically related outputs. The left-to-right directionality is sacrificed only when it would result in output forms that involve marked sequences or toneme deletion at the prominent edge of a tone, which are forbidden by the markedness constraint OCP-T and the positional IO-faithfulness constraint Max-IO-t-R, respectively. Thus the rule application directionalities in Tianjin are naturally predicted by the interaction of Ident-BOt, OCP-T, and Max-IO-t-R, where Ident-BOt must be dominated by the latter two constraints.