  We describe the positive cone generated by bigraded Betti diagrams of
artinian modules of codimension two, whose resolutions become pure of a given
type when taking total degrees. If the differences of these total degrees, p
and q, are relatively prime, the extremal rays are parametrised by order ideals
in N^2 contained in the region px + qy < (p-1)(q-1). We also consider some
examples concerning artinian modules of codimension three.
