 We are unable to reconstruct exactly what Reichenbach may have intended, in particular since his definition of lack of aftereffect is difficult to distinguish from the criterion of invariance under subsequence selection. But we believe it is something close to the following: A sequence of Bi is “free from aftereffect” if (i) a subsequence is selected based on a rule of the form “For each index i in the sequence, include the (i+k)th element in the sequence if the ith element is B (or ~B),” (ii) if the subsequence thus selected has the same event probabilities as the original sequence, and (iii) if this holds for all lags k &gt; 0. If our reconstruction is correct, this would distinguish Reichenbach’s account of lack of aftereffect from that of invariance under subsequence selection, because the former includes subsequence selection rules that depend on the values of certain items in the sequence, while the latter includes only rules that are based on the indices. Two more definitions are required for the full picture. First, a selection S of a subsequence of sequence A belongs to the “domain of invariance” of B, if the probability of B (for all lags) in S is unchanged from the probability of B in A, and if the same holds for any selection S from A with a lag. Second, a subsequence selected by an algebraic rule that partitions the sequence A—e.g. take every fourth element (see p. 144 for details)—is called a “regular division” of A. Putting the pieces together, Reichenbach requires for the “normality” of a sequence A that it be free of aftereffect and that all “regular divisions” of A are in the domain of invariance of B. This condition of regular divisions seems to underlie the stationarity expressed in the second axiom of order. 