There are, however, some related observations. One is that if one takes seriously the classical picture of evolution off of surfaces with different values of x + , one finds that the surfaces with x + ≠ 0 are only invariant under a six parameter subgroup. This means that if one chooses a quantization surface with a fixed non-zero value of x + , the resulting quantum theory would require a fourth interacting generator. This does not happen in light-front quantum mechanics; all seven kinematic generators remain kinematic. The reason is that the choice of light front is more closely related to the choice of kinematic subgroup, than the choice of an initial value surface.