The explanation of coordinate structures is one of the greatest challenges for any theory of language. A prerequisite to any successful explanation is a careful and accurate description of coordinate structures in many different languages. This work provides such an account for the Yaqui language, a member of the Uto-Aztecan family, spoken mainly in the north of Mexico, and is the result of several years of investigation. It explores and describes the patterns of sentence coordination, verbal coordination, and nominal coordination. The sentence coordination data raise fundamental problems for theories which suggest that coordinators are heads of their own projection. The problem arises from the several positions that a coordinator can occupy in the coordinated structure.

The author proposes that a viable explanation is to take the coordinating particles as having the function of licensing adjunction processes. In other words, to coordinate is to adjoin. The proposal is extended to verbal and nominal coordination. The nominal coordinated structures show number agreement conflicts with the verb that again are theoretically challenging, and a solution is proposed within the framework of Optimality Theory.