The simplest form of the Leggett–Garg inequality derives from examining a system that has only two possible states. These states have corresponding measurement values Q = ± ± 1 . The key here is that we have measurements at two different times, and one or more times between the first and last measurement. The simplest example is where the system is measured at three successive times t 1 < t 2 < t 3 . Now suppose, for instance, that there is a perfect correlation C 13 = 1 between times t 1 and t 3 . That is to say, that for N realisations of the experiment, the temporal correlation reads