AbstractIn this paper, I argue that there are fundamental common features shared by interaction and grammar that suggest some kind of interdependence between the two and a nonautonomy of the language faculty; one of these fundamental common features is that of ‘projectability’. Human interaction rests on the possibility of projection; the grammars of human languages provide interlocutors with sedimentated and shared ways of organizing them.Empirical evidence for this view comes from structural homologies between action projection and grammatical projection. In particular, it is shown (on the basis of conversational examples from German) that in both cases: (1) the strength of the projective force can vary; (2) emerging gestalts can be framed by weakly projecting introductions; (3) projections can be interrupted; (4) full gestalts can be expanded; and (5) projections can be abandoned.The projectability of syntactic as well as sequence structures suggests an approach to syntax that takes into account the temporal unfolding of speech in time. It also suggests looking at the indeterminate areas between syntax and conversational structure in which the borderline between grammar and interaction can only be drawn on an ad hoc basis.