Gives ranges, construction, performances, output characteristics and testing of rotary displacement gas meters (hereinafter referred to as RD meters or simply meters) for gas volume measurement. Relevant to rotary displacement gas meters used to measure the volume of at least fuel gases of the 1st, 2nd and 3rd gas families, the composition of which is specified in EN 437, at a maximum working pressure up to and including 20 bar over an ambient and gas temperature range of at least -10 degree C to +40 degree C.