Abstract: In this work we study 2 x 2 hyperbolic systems of the form Ut + A(U)Ux + B(U)Uy = 0 with degenerate characteristic structure. We define partially aligned systems as those for which A and B have a common eigenvector, and we show that the characteristic structure degenerates into a pair of curves if and only if the system is partially aligned. We describe examples and some basic properties of such systems Ver menos