Loudness ratings of local telephone circuits are essential quantities in planning national telephone networks and in co-ordinating international telephony. At present, these have to be obtained by subjective testing, which is laborious, expensive and not readily reproducible except in particularly well equipped laboratories. Methods are also available (and in use for certain purposes) for obtaining loudness ratings by calculations and by instrumental measurements, but, to ensure uniform results, it is necessary to secure agreement on precise definitions of certain items so that the principles can be implemented equally well, to yield the same numerical values, by calculation and by objective instrumentation as well as subjectively. A new set of definitions is proposed. These are similar to those already examined and adopted provisionally by the British Post Office. Further examination of certain aspects is being carried out in the CCITT laboratory by experiments which include subjective tests, calculations based on frequency-response measurements with artificial mouths and artificial ears and by objective instrumentation which uses a special signal generator and a voltmeter. Results are showing that, with suitable precautions, the ratings can be made practically independent of the method of implementation. An important feature of such a unified set of principles will be the possibility of making confident theoretical estimates of the performance of hypothetical local telephone circuits which might later be realised in service. Thus, design, development, acceptance testing and maintenance of telephone equipment could, probably for the first time, be based on common fundamental principles.