The breakage of material in cone type gyratory crushers is traditionally regarded as relying upon single particle breakage. In the last ten years the emphasis has shifted with manufacturers trying to generate higher degrees of interparticle breakage. Increasing the degree of interparticle crushing is claimed to improve crushing efficiency and product shape. The current study uses form conditioned crushing tests (geometry controlled compression) to investigate how multiple particles respond to crushing loads. By, variation of test parameters the breakage characteristics of a rock material can be determined and compared to traditional single particle crushing. The selection function, S (probability of crushing a single particle), seems to be related to the ratio between stroke and bed height, s/b, with a second order polynomial in s/b. An analysis of a given crusher chamber gives selection values in the range 0.05 < S < 0.4. Given the geometry of this chamber it is clear that much of the breakage will be interparticle. However, the selection values indicate that the efficiency of crushing is poor. Using the approach outlined a mechanistic crusher model has been developed. The model seeks to describe the crushing process in relation to the machine operating parameters, chamber geometry and the material characteristics of the feed. In this way predictions of material flow and product size gradation are obtained that can be used to improve the understanding and design of crushers.