The directives relating to 500/20-Hz signalling sets are provisional. An Administration intending to purchasenew signalling sets for use on international circuits which for the time being are to be operated on a manual basis, mayfind it advantageous, by agreement with the Administrations interested in the operation of the circuits concerned, to usesignal receivers and transmitters conforming to the specifications for international automatic equipment. This will permit a greater technical uniformity of installations and will avoid having to replace the signal receivers when,ultimately, these circuits are operated on an automatic or semi-automatic basis.