THE technique of modern radio communication demands a very high degree of precision in the control and measurement of frequency. Considerable attention is therefore devoted by the more important national administrations to the development and maintenance of accurate frequency standards. In Great Britain one of the standards installed at the National Physical Laboratory provides a frequency of 1,000 cycles per second, the stability of which is better than one part in ten million. In order that this standard may be compared with those of other countries, the derived alternating current is used to modulate the carrier wave of a radio transmitting station. At the distant receiving station the modulation is extracted from the arriving signal and its frequency is compared with that of the local standard. In this manner frequency comparison measurements maybe carried out simultaneously in different countries. Under the auspices of the Union Radio Scientifique Internationale, and with the co-operation of the British Broadcasting Corporation, such an international frequency comparison will be carried out during the night of March 12–13 next. On this occasion, the output from the frequency standard at the N.P.L. will be employed to modulate waves from the Droitwich, Scottish National and Scottish Regional stations of the B.B.C., simultaneously with the frequency of 1,000 cycles per second, for a period of about an hour and a half. The object of using several stations is to enable observations to be made on two or three carrier frequencies simultaneously, so that the effect of fading phenomena on frequency stability may be studied. On the same night a special emission of a constant frequency of five million cycles per second will be made from the U.S. Bureau of Standards, Washington, of sufficient intensity for satisfactory reception in Europe. Persons and organisations desirous of making use of any of these emissions may obtain further details from Dr. E. H. Rayner, president of Commission I of the U.R.S.I., at the National Physical Laboratory, Teddington, Middlesex.