In many applications time passes between trials, as the index i increases. In effect, the trials "X"1, "X"2, ... "X"i, ... happen at "points in time" 1, 2, ..., "i", ... That passage of time and the associated notions of "past" and "future" are not necessary, however. Most generally, any "X"i and "X""j" in the process are simply two from a set of random variables indexed by {1, 2, ..., "n"}, the finite cases, or by {1, 2, 3, ...}, the infinite cases.