The inter-Allied cryptologic collaboration prevented duplication of effort and facilitated discoveries. Before fighting had started in Norway in April 1940, the Polish-French team solved an uncommonly hard three-letter code used by the Germans to communicate with fighter and bomber squadrons and for exchange of meteorological data between aircraft and land. The code had first appeared in December 1939, but the Polish cryptologists had been too preoccupied with Enigma to give the code much attention. With the German assault on the west impending, however, the breaking of the "Luftwaffe" code took on mounting urgency. The trail of the elusive code (whose system of letters changed every 24 hours) led back to Enigma. The first clue came from the British, who had noticed that the code's letters did not change randomly. If "A" changed to "P", then elsewhere "P" was replaced by "A". The British made no further headway, but the Poles realized that what was manifesting was Enigma's "exclusivity principle" that they had discovered in 1932. The Germans' carelessness meant that now the Poles, having after midnight solved Enigma's daily setting, could with no further effort also read the "Luftwaffe" signals.