Consider NMO from time t'' to traveltime depth t' by t''2= t'2 + x2/v22, followed by another NMO transform which uses the transformation equation t'2= t2 + x2/v12. Show that the overall transformation is another NMO transformation. What is its velocity? Notice that cascaded NMO can be used to correct an NMO velocity. Thus it can be called residual velocity analysis or residual normal moveout.