Abstract It is a distinctive feature of OT (Prince and Smolensky, 1993) to allow for the co-existence in one grammar of perfectly opposite constraints. This paper shows that rather than being the uninteresting OT-equivalent of the parameters of the Government and Binding tradition (Chomsky, 1981), such constraint-pairs make crucial distinctions among the two frameworks. With this goal in mind, I examine the Chadic language Kanakuru (Tuller, 1992), arguing that leftward and rightward structural focus alternate in systematic ways. I then show how such a mixed pattern follows precisely from the existence in UG of two constraints specifying opposite alignment-directions for structural contrastive focus. The same constraints are also responsible for the uniform focus patterns of languages like Podoko and Italian (Tuller, 1992; Belletti and Shlonsky, 1995; Samek-Lodovici, 1996, 1997a).