Ch 24.5 The Clauser-Horne-Shimony-Holt generalization of Bell's inequality: $|S| \leq 2$, where $S = E(a,b) - E(a,b') + E(a',b) + E(a',b')$ involves correlations $E$ at four combinations of measurement settings. Quantum mechanics allows $|S| = 2\sqrt{2} \approx 2.83$ (Tsirelson's bound). *See also:*