Richard Dolan, David Horn and Christoph Schmid, in 1967, will develop rules for hadron exchanges. A particle and an antiparticle will scatter, and, virtual particles can be exchanged in one of two ways:
S-channel:
The two particles will annihilate, and fall apart into their final states.
T-channel:
The particles will exchange states by emission or absorption.
It is going to be discovered by theorists that these two ways particles were exchanging, included each other and could be used together to describe the amplitude as a whole.