  The basic idea of Bell's theorem is as follows. A model of the EPR/B experiment postulates that the state of the particle pair together with the apparatus settings to measure (or not to measure) certain spin properties determine the probabilities for single and joint spin-measurement outcomes. A local Bell model of this experiment also postulates that probabilities of joint outcomes factorize into the single probabilities of the L- and the R- outcomes: The probability of joint outcomes is equal to the product of the probabilities of the single outcomes. More formally, let λ denote the pair's state before any measurement occurs. Let l denote the setting of the L-measurement apparatus to measure spin along the l-axis (i.e., the l-spin of the L-particle), and let r denote the setting of the R-measurement apparatus to measure spin along the r-axis (i.e., the r-spin of the R-particle). Let xl be the outcome of a l-spin measurement in the L-wing, and let yr be the outcome of a r-spin measurement in the R-wing; where xl is either the L-outcome l-spin ‘up’ or the L-outcome l-spin ‘down,’ and yr is either the R-outcome r-spin ‘up’ or the R-outcome r-spin ‘down.’ Let Pλ l r(xl &amp; yr) be the joint probability of the L- and the R-outcome, and Pλ l(xl) and Pλ r(yr) be the single probabilities of the L- and the R-outcome, respectively; where the subscripts λ, l and r denote the factors that are relevant for the probabilities of the outcomes xl and yr. Then, for any λ, l, r, xl and  yr:[3]