From the inception of the RAF in 1918 the new service developed traditions fast. In particular, its squadrons assumed a combination of heraldic devices and colourful markings with which to identify machines and inspire men. With the potential onset if war in 1938, the very public and well-known unit markings were replaced by two-letter codes, designed to confuse the enemy. Through the uneasy period leading to the formal declaration of war, these codes became well-known locally and there was a second set for application with the onset of war. As the war progressed the unit codes expanded to incorporate numerals and the system expanded to the USAFF and the Commonwealth and the Allied units. To preserve secrecy the codes were only ever listed in a series of top-secret documents and the previous set destroyed. The authors have painstaking researched the codes and building on from earlier works have produced the definitive work on the subject.