  Observers in different inertial frames can see a set of spacelike separated
events as occurring in different orders. Various restrictions are studied on
the possible orderings of events that can be observed. In 1+1-dimensional
spacetime {(1 2 3), (2 3 1), (3 1 2)} is a disallowed set of permutations. In
3+1-dimensional spacetime, any four different permutations on the ordering of n
events can be seen by four different observers, and there is a set of four
events such that any of the 4!=24 possible orderings can be observed in some
inertial reference frame. A more complicated problem is that of five observers
and five events, where of the 7,940,751 choices of five distinct elements from
S_5 (containing the identity), all but at most one set of permutations can be
realized, and it is shown that this remaining case is impossible. For six
events and five observers, it is shown that there are at least 7970 cases that
are unrealizable, of which at least 294 do not come from the forbidden
configuration of five events.
