This Recommendation deals with: I: hot cathode tubes and valves. Ia: gasfilled rectifier tubes and valves. Ib: thyratrons except for those used for pulse modulation. II: cold cathode tubes. IIa: voltage stabilizing tubes, excluding corona stabilizers. IIb: trigger tubes. This Recommendation is based on current practice of the measurement of gasfilled tubes and valves. It should not be regarded as a Recommendation in the sense of a standard, because a more detailed description of the methods of measurement is needed if measuring results on the basis of these principles have to be comparable within definite tolerances.