Depending on what we count as an occurrence of two distinct words used together, the two-word stage can begin around eighteen to twenty months, as the child’s vocabulary moves beyond fifty words. By the time the child is two years old, a variety of combinations, similar to baby chair, mommy eat, cat bad, will usually have appeared. The adult interpretation of such combinations is, of course, very much tied to the context of their utterance. The phrase baby chair may be taken as an expression of possession (= this is baby’s chair), or as a request (= put baby in chair), or as a statement (= baby is in the chair), depending on different circumstances.

Whatever it is that the child actually intends to communicate through such expressions, the significant functional consequences are that the adult behaves as if communication is taking place. That is, the child not only produces speech, but also receives feedback confirming that the utterance worked as a contribution to the interaction. Moreover, by the age of two, whether the child is producing 200 or 300 distinct “words,” he or she will be capable of understanding five times as many, and will typically be treated as an entertaining conversational partner by the principal caregiver.