Communication Accommodation Theory (CAT) predicts that people adjust their language to match that of the other to promote comprehension, coordinate action, and facilitate harmonious relationships. CAT predicts that mothers will adjust their sentence length and complexity to match those of children. Prior tests of CAT confounded trait-like language use with accommodation; we refine the concept of accommodation using the theoretical logic of the Social Relations Model (SRM). We argue that (a) trait effects should be partitioned from unique relationship effects, (b) relationship effects are the appropriate estimate of accommodation, and (c) relationship (dyadic) reciprocity is the mechanism of accommodation. We tested our proposal in 152 mother-child dyads who interacted in play sessions. Mother and child language was recorded, transcribed, and coded. SRM revealed stable, trait-like individual differences in language used by each; however, this stability was much stronger among children than mothers. Both made unique reciprocal linguistic adjustments when interacting with specific adults or children. These results support a new theoretical perspective on communication accommodation and adult child-directed language.