# graph theory analogue of rectangular matrix

Graphs are usually defined as a set of vertices $V$ together with a set of edges $E$ consisting of elements $V \times V$. I'm interested in a slight generalization of this, where instead one has two sets $V$, $W$ and the edges are taken from $V \times W$. The adjacency matrix of such an object would be rectangular, as opposed to the adjacency matrix of a regular old graph, which is square. Is there a name for this?

I'm interested in this because I'm writing a data structure to represent a graph in terms of its partitioning into sub-graphs for the purposes of parallel computation. For example, given a graph $G$ on a vertex set $V$, we can partition $V$ into disjoint subsets $V_1$ and $V_2$; $G$ is then naturally divided into two subgraphs $G_1$ and $G_2$ describing connections among $V_1$ and $V_2$ respectively, and a "rectangular graph" $H$ describing connections between $V_1$ and $V_2$. I'd like to give the class a name that makes sense.