The results of early international comparisons of frequency are briefly discussed. In measurements carried out in 1925 the agreement obtained was 0.1 per cent, and in 1929 an agreement of 0.006 per cent was realized. At this time the frequencies of the standards at several laboratories could be determined absolutely with an accuracy greater than that with which the comparisons could be effected.The nature of the recent modulated emissions is described and the results of three of the emissions are given. In two of the comparisons, measurements made at three different laboratories are in agreement within 1 part in 107An experiment is described showing that the frequency of modulation of an aerial current can be controlled by a distant standard with an accuracy of at least 2 parts in 108; and that the frequency measurements can be effected with this order of accuracy.