= Quark rephasing and CP conservation
For nondegenerate quark masses, independent phase changes of the up- and down-quark mass eigenfields alter the numerical <CKM matrix> by row and column phases without changing physics. Thus <CP symmetry> requires that some allowed phase convention makes the matrix real, rather than requiring real entries in every convention. With $N$ generic families, removing $2N-1$ phases from a unitary matrix leaves $(N-1)(N-2)/2$ physical CP-violating phases and $N(N-1)/2$ mixing angles.
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