1. the quality of a relationship between two people, objects, or situations such that the qualities of one supplement or enhance the different qualities of the others.

2. in any dyadic relationship, the existence of different personal qualities in each of the partners that contribute a sense of completeness to the other person and provide balance in the relationship. Certain behaviors or traits tend to elicit the complementary behaviors or traits from partners (e.g., dominance often is followed by submission, and vice versa). Complementarity is an important factor in building rapport and the therapeutic relationship in psychotherapy because it reduces anxiety, preserves self-concept, and reinforces the interaction pattern. —complementary adj.