We construct a combinatorial model that is described by the cube recurrence, a quadratic recurrence relation introduced by Propp, which generates families of Laurent polynomials indexed by points in ${\Bbb Z}^3$. In the process, we prove several conjectures of Propp and of Fomin and Zelevinsky about the structure of these polynomials, and we obtain a combinatorial interpretation for the terms of Gale-Robinson sequences, including the Somos-6 and Somos-7 sequences.  We also indicate how the model might be used to obtain some interesting results about perfect matchings of certain bipartite planar graphs.
['Gabriel D. Carroll', 'David E Speyer']
The Cube Recurrence