 The only currently discussed proposal is based on a theorem of Malament (1977), who argues that standard synchrony is the only simultaneity relation that can be defined, relative to a given inertial frame, from the relation of (symmetric) causal connectibility. Let this relation be represented by κ, let the statement that events p and q are simultaneous be represented by S(p,q), and let the given inertial frame be specified by the world line, O, of some inertial observer. Then Malament’s uniqueness theorem shows that if S is definable from κ and O, if it is an equivalence relation, if points p on O and q not on O exist such that S(p,q) holds, and if S is not the universal relation (which holds for all points), then S is the relation of standard synchrony.