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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]

Nicola Cabibbo organized decay probability amplitudes of the weak interaction through a single mixing angle. His 1963 paper related strangeness-conserving and strangeness-changing processes within a symmetry-based weak current. In modern quark notation, the combination $d^{\prime }=d\cos {\theta }_C+s\sin {\theta }_C$ expresses this quark mixing: the relative strengths depend on one Cabibbo angle. Comparing different decay rates therefore tests one common structure instead of fitting every weak channel independently. The quark notation is a later interpretation of the original current construction.^[ref]

A physicist working on quark mixing.^[ref]