Lev Landau proposed in 1957 CP-symmetry, often called just CP as the true symmetry between matter and antimatter. CP-symmetry is the product of two transformations : C for charge conjugation and P for parity. In other words, a process in which all particles are exchanged with their antiparticles was assumed to be equivalent to the mirror image of the original process and so the combined CP-symmetry would be conserved in the weak interaction.