to quantum mechanics, the results of the two measurements are opposite to one another: one measurement yields $+\frac12$ and the other $-\frac12$ [@NC Box 2.7]. This is true even if each measurement is performed outside of the lightcone of the other and thus cannot possibly affect the other measurement.

Alternatively, one can work with photons, which are spin 1 particles. In this connection, see Figure 1 which, together with the caption, is borrowed from [@Aspect].

The two photons are moving along the $z$ axis. Formula ${\ensuremath{|\Psi(1,2)\rangle}}$ in the figure is $\frac1{\sqrt2} \Big({\ensuremath{|00\rangle}} + {\ensuremath{|11\rangle}}\Big)$ where 0 and 1 are unit vectors in the Hilbert space for the quantum system of one photon. 0 and 1 correspond to polarization in the directions of the $x$ and $y$ axes respectively. If the orientations $\bf a, \bf b$ of the analyzers in the figure coincide then the two measurement outcomes are guaranteed to coincide: both are $+1$ or both are $-1$.

Einstein, Podolsky and Rosen (EPR) saw contextuality as an indication that quantum mechanics is incomplete, i.e., that some hidden variables are missing [@EPR]. Bell famously proved that local hidden-variable theories contradict quantum-mechanical statistics [@Bell]. Subsequent experiments supported the latter.

In general, the 