An aircraft registration is a unique alphanumeric string that identifies a civil aircraft, in similar fashion to a licence plate on an automobile. In accordance with the Convention on International Civil Aviation all aircraft must be registered with a national aviation authority and they must carry proof of this registration in the form of a legal document called a Certificate of Registration at all times when in operation. Most countries also require the aircraft registration to be imprinted on a permanent fireproof plate mounted on the fuselage for the purposes of post-fire/post-crash aircraft accident investigation. Because aeroplanes/airplanes (see British and American spelling differences) typically display their registration numbers on the aft fuselage just forward of the tail (more often on the tail itself, in earlier times) the registration is often referred to as the “tail number”. Although each aircraft registration is unique, some countries allow it to be re-used when the aircraft has been sold, destroyed or retired. For example, N3794N is assigned to a Mooney M20F. It had been previously assigned to a Beechcraft Bonanza (specifically, the aircraft in which Buddy Holly was killed). Also note that an individual aircraft may be assigned different registrations during its existence. This can be because the aircraft changes ownership, jurisdiction of registration, or in some cases for vanity reasons.