X-ray beams are usually described by "MV" numbers which represent accelerating potentials (AP) and approximations to the maximum energies in the photon spectra. However, these numbers do not uniquely specify the properties of the beams. Current high energy photon dosimetry protocols specify radiation quality in terms of a measured ionization ratio which is equivalent to the ratio of the tissue-maximum ratios at depths 10 cm and 20 cm, for field size 10 cm X 10 cm [TMR)20(10]. For convenience, the American Association of Physicists in Medicine introduced a new parameter, known as the Nominal Accelerating Potential (NAP), which was derived from (TMR)20(10) and features values in MV units that are similar to those of the conventional accelerating potentials. (TMR)20(10) and Nominal Accelerating Potential may be considered to be expressions of the penetrating powers of x-ray beams. We determined (TMR)20(10) and Nominal Accelerating Potential for 460 treatment machines with stated accelerating potentials from 4 MV to 25 MV in the Quality Assurance Review Center's files of machine data from institutions that participate in cooperative clinical trials. The results demonstrate appreciable variability of the two parameters at each stated accelerating potential, with overlapping of adjacent groups of machines. It is concluded that the manufacturers' MV numbers do not reliably identify x-ray beams in terms of their depth dose properties. To promote standardization and consistency of energy specification in clinical trials as well as in general practice, we propose that x-ray beams be designated by their Nominal Accelerating Potential values as an adjunct to the use of (TMR)20(10) in radiation therapy.