Like all fields of human artistic endeavour, music is constrained by our cognitive processing requirements and limitations (Swain, 1986; Lerdahl, 1988; Huron, 2001). This articleconsiders particular forms of constraint pertaining to the relationships that the structure-seeking mind (subconsciously) fabricates between perceived musical events. It is proposed that 2±1 may be a universal limitation pertaining to the level of relationships so ideated. That is, in terms of Lewin'S (1987) theoretical framework in which “intervals” can be intuited between the “elements” of musical “spaces”, it is posited that the cognition of musical structure occurs either through intervals (level 1), through intervals between these (level 2), or — in some circumstances — through intervals between these(level 3). This proposition is explored through the psychomusicological model developed by Ockelford (1991, 1993, 1999), which too analyses musical structure in terms of the relationships that may be cognised between its discrete perceptual components. In particular, the model identifies a type of cognitive link through which events (at any level) arefelt to imply others the same or similar — through so-called “zygonic” relationships. This theory suggests a further general principle: that the highest level of relationship inoperation at any given point must be zygonic if the music is to be structurally coherent. Evidence for this, and for the limit on the level of relationships of 2±1, is offeredthrough a series of musical examples, which illustrate a variety of musical organisation in action. Finally, empirical work is suggested to explore further the theoretical ideas that arepresented here.