= Solution
The Standard Model has two independent sources of <CP violation>. Weak CP violation comes from the irreducible phase of the <Cabibbo-Kobayashi-Maskawa matrix>; its basis-independent measure is the <Jarlskog invariant>. Strong CP violation is governed by
$$
\bar\theta=\theta_{\rm QCD}+\arg\det(Y_uY_d),
$$
which multiplies the <Yang-Mills theta term> $G_{\mu\nu}^a\widetilde G^{a\mu\nu}$. Quark chiral rephasings shift $\theta_{\rm QCD}$ and the mass-matrix phase oppositely, leaving $\bar\theta$ invariant. The CKM phase and $\bar\theta$ are otherwise independent parameters: observed weak CP violation does not explain why neutron-electric-dipole bounds require $|\bar\theta|\ll1$. This unexplained smallness is the <Strong CP problem>.
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