  We clarify the relation between the variable MT2 and the method of kinematic
constraints, both of which can be used for mass determination in events with
two missing (dark matter) particles at hadron colliders. We identify a set of
minimal kinematic constraints, including the mass shell conditions for the
missing particles and their mother particles, as well as the constraint from
the measured missing transverse momentum. We show that MT2 is the boundary of
the mass region consistent with the minimal constraints. From this point of
view, we also obtained a more efficient algorithm for calculating MT2. When
more constraints are available in the events, we can develop more sophisticated
mass determination methods starting from the MT2 constraint. In particular, we
discuss cases when each decay chain contains two visible particles.
