Abstract: We study shock formation for the 2 x 2 system u1 + (u²)x + (uv)y = 0 and vt + (uv)x + (v²)y = 0. Our result is based on the argument, due to J.Keller and L.Ting [?], about the evolution along a characteristic of the compression rate of nearby characteristics. This system is one of a class of systems, called partially aligned, which exhibit a degenerate characteristic structure where a pair of directions substitutes the usual cone at every point. Our analysis can be extended to a set of partially aligned systems satisfying a sharp algebraic constraint Ver menos