During the Second World War, the
British Commonwealth Air Training Plan (BCATP) set out co-operation between the various
British Commonwealth air forces. Under Article XV of the BCATP, RCAF squadrons were attached to and/or formed within British
RAF operational formations. These squadrons were known as '
Article XV squadrons'. To avoid confusion with squadron numbers of other Commonwealth nations, RCAF units were assigned squadron numbers from 400 to 449. Not all squadrons up No. 449 were formed, however. This squadron series continued through the post-war years, and the majority of current
Royal Canadian Air Force operational squadrons are still numbered in the 400s due to their legacy as wartime units. For this historical reason current squadrons are listed here with their original RCAF names. ==600-series squadrons==