Squadrons are the main form of flying unit of the Royal Air Force (RAF). These include Royal Flying Corps (RFC) and Royal Naval Air Service (RNAS) squadrons incorporated into the RAF when it was formed on 1 April 1918, during the First World War. Other squadrons of the RAF include those from Commonwealth air forces which have served within the RAF structure and squadrons of the Fleet Air Arm before it transferred to the Royal Navy in 1939. Some Squadron names include the location they were originally formed.