describes the exchange of a transverse photon. It has a polarization vector e λ λ and couples to a particle with charge q and three-momentum p with a strength − − q 4 π π e λ λ ⋅ ⋅ p / m . Since e λ λ ⋅ ⋅ k = 0 in this gauge, it doesn't matter if one uses the particle momentum before or after the photon couples to it.