  Three-step cascade decays into two invisible particles and two visible
particles via two intermediate on-shell particles develop cusped peak
structures in several kinematic distributions. We study the basic properties of
the cusps and endpoints in various distributions and demonstrate that the
masses of the missing particles and the intermediate particles can be
determined by the cusp and endpoint positions. Effects from realistic
considerations such as finite decay widths, longitudinal boost of the parent
particle, and spin correlations are shown to be under control for the processes
illustrated.
