  Quantum mechanics exhibits a very peculiar form of contextuality. Identifying
and connecting the simplest scenarios in which more general theories can or
cannot be more contextual than quantum mechanics is a fundamental step in the
quest for the principle that singles out quantum contextuality. The former
scenario corresponds to the Klyachko-Can-Binicioglu-Shumovsky (KCBS)
inequality. Here we show that there is a simple tight inequality, twin to the
KCBS, for which quantum contextuality cannot be outperformed. In a sense, this
twin inequality is the simplest tool for recognizing fully contextual quantum
correlations.
