The authors captured mother-child and father-child relationships when children were 7 and 15 months old by coding 4 explicitly dyadic components of mutually responsive orientation (MRO): coordinated routines, harmonious communication, mutual cooperation, and emotional ambiance. These components were coded in 102 families in naturalistic contexts at home and in the laboratory. Using confirmatory factor analytic methods, the authors demonstrated that MRO (a) is a unidimensional latent construct, (b) can be distinguished from and is not reducible to the 2 interacting individuals' qualities (responsiveness and positive affect), and (c) has remarkable measurement equivalence from ages 7 to 15 months for both mother-child and father-child relationships and has remarkable measurement equivalence across both parent-child relationships at both child ages. This study represents an effort to advance the science of relationships by moving away from individual-based measures to dyadic measures and by using contemporary data analytic strategies.