The consistent histories interpretation of quantum mechanics was introduced by Griffiths (1984), and discussed by Omnès in a series of papers beginning with (Omnès 1988). The decoherent histories approach that first appeared in Gell-Mann &amp; Hartle (1990) contains similar ideas. The single term “histories” will be employed below, with the understanding that at points where the “consistent” and “decoherent” approaches might be slightly different, the former is intended.