The Standard model of particle physics has three related natural near-symmetries. These state that the actual universe about us is indistinguishable from one where:

-Every particle is replaced with its antiparticle. This is C-symmetry (charge symmetry);

-Everything appears as if reflected in a mirror. This is P-symmetry (parity symmetry);

-The direction of time is reversed. This is T-symmetry (time symmetry).

These symmetries are near-symmetries because each is broken in the present-day universe. However, the Standard Model predicts that the combination of the three (that is, the simultaneous application of all three transformations) must be a symmetry, called CPT symmetry. CP violation, the violation of the combination of C- and P-symmetry, is necessary for the presence of significant amounts of baryonic matter in the universe. CP violation is a fruitful area of current research in particle physics.