in place of P ⋅ ⋅ p = 0 . This conforms with the earlier claim on the vanishing of the relative energy in the c.m. frame made in conjunction with the TBDE. In the second choice the c.m. value of the relative energy is not defined as zero but comes from the original generalized mass shell constraints. The above equations for the relative and constituent four-momentum are the relativistic analogues of the non-relativistic equations p → → = m 2 p → → 1 − − m 1 p → → 2 M , p → → 1 = m 1 M P → → + p → → , p → → 2 = m 2 M P → → − − p → → .