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The Cabibbo angle ${\theta }_C$ describes the real two-generation quark mixing rotationThus $V_{ud}=V_{cs}=\cos {\theta }_C$, $V_{us}=\sin {\theta }_C$ and $V_{cd}=-\sin {\theta }_C$. Nicola Cabibbo formulated the mixing of weak charged current couplings before the three-generation mechanism of CP violation; this foundational contribution is distinct from the later extension.^[ref]