[en] The purpose of this note is to bring together the various assumptions that are stated in the literature or are tacitly understood and which form the basis for the well known relationship between the grade-thickness product and the total or gross gamma ray response of a borehole probe. As this relation pertains to calibration, certain conditions have been established by the Energy Research and Development Administration (ERDA) in order to assure uniformity among the various gross gamma ray probes. That these conditions and their implications are sometimes misunderstood is an understatement. An example illustrating the analysis of digital calibration data is presented in the hope that such misunderstanding may be cleared up and at the same time demonstrate the efficacy of a few common integration techniques