On the conception of time congruence as conventional, the preference for the customary definition of isochronism—a preference not felt by Einstein in the general theory of relativity (GTR), as we shall see in Section B—can derive only from considerations of convenience and elegance so long as the resulting form of the theory is not prescribed. Hence, the thesis that isochronism is conventional precludes a difference in factual import (content) or in explanatory power between two descriptions one of which employs the customary isochronism while the other is a "translation" (transcription) of it into a language employing a time congruence incompatible with the customary one. As a test case for this thesis of explanatory parity, the general outline of a counter-argument has been suggested which we shall be able to state after some preliminaries.