Constraint dynamics applied to the TBDE requires a particular form of mathematical consistency: the two Dirac operators must commute with each other. This is plausible if one views the two equations as two compatible constraints on the wave function. (See the discussion below on constraint dynamics.) If the two operators did not commute, (as, e.g., with the coordinate and momentum operators x , p ) then the constraints would not be compatible (one could not e.g., have a wave function that satisfied both x Ψ Ψ = 0 and p Ψ Ψ = 0 ). This mathematical consistency or compatibility leads to three important properties of the TBDE. The first is a condition that eliminates the dependence on the relative time in the center of momentum (c.m.) frame defined by P = p 1 + p 2 = ( w , 0 → → ) . (The variable w is the total energy in the c.m. frame.) Stated another way, the relative time is eliminated in a covariant way. In particular, for the two operators to commute, the scalar and four-vector potentials can depend on the relative coordinate x = x 1 − − x 2 only through its component x ⊥ ⊥ orthogonal to P in which x ⊥ ⊥ μ μ = ( η η μ μ ν ν − − P μ μ P ν ν / P 2 ) x ν ν , P μ μ x ⊥ ⊥ μ μ = 0.