In the Standard Model where one deals with six quarks whose masses are described by the Yukawa matrices Y u and Y d , the physical CP violating angle is θ θ ¯ ¯ = θ θ − − arg ⁡ ⁡ det ( Y u Y d ) . Since the θ-term has no contributions to perturbation theory, all effects from strong CP violation is entirely non-perturbative. Notably, it gives rise to a neutron electric dipole moment