Introduction: Ternary stress patterns have posed a problem for a parametric metrical theory for some time. In this paper, it is argued that ternary systems can be derived in an explanatory fashion from binary systems. The basic idea is that ternary stress systems can be analyzed as binary stress systems if the theory of extrametricality is enriched. Two specific proposals regarding extrametricality are made. First, extrametricality must be tolerated not just at the edge of morphological and syntactic constituents, but also at the edge of phonological constituents. Second, extrametricality can be lost if adjacent feet are subminimal. The organization of this paper is as follows. First, the foot typology is briefly reviewed. Then the theory of extrametricality is presented. It is argued that regardless of the analysis of ternary systems, the theory of extrametricality must be enriched as outlined above. Four metrical systems are then considered: Cayuvava, Chugach, Winnebago, and Estonian. Each of these systems provides arguments for deriving ternarity as proposed here.