Scattering t-channel ($t=(p_1-p_3{)}^2$)

With a specified ordering of external legs, this crossed channel carries the difference between an incoming and an outgoing energy-momentum vector. A one-particle exchange pole requires that difference to be on the exchanged particle's mass shell. Ordered amplitudes in one spatial dimension can label the crossed invariant using the difference of the two rapidity-parameterized external momenta; the external-leg convention fixes which crossed invariant is named $t$.