All voice substitutions, including voice substitutions that do not express meaning, are directed events, and this direction is directed from the left member to the right member of the substitution. By distinguishing one of the many variants of each morpheme as an output form or an initial form, a morpho-phonological description can be systematically presented. Putting the issue in this way can shed light on the solution to the problem of determining the initial form and direction of change. This problem is discussed in the article.