There is an important distinction to be drawn in the way different kinds of motivic relationships are perceived. Some relationships are detected quickly and automatically; other kinds are detected (if at all) only slowly and deliberately. There is a phenomenological difference here as well. These differences are nicely accounted for by Jerry Fodor's theory of modularity. It is argued that certain relationships are perceived in a "modular" fashion, and others are not. It is hypothesized that the relationships perceived in a modular way are those between segments that are (a) related by tonal transposition and (b) parallel relative to the metrical structure. This view accounts for the differences between perception of different kinds of relationships and also sheds light on metrical structure in general, the "rehearing" problem, and the issue of "mandatoriness" in musical perception.