In accordance with the Convention on International Civil Aviation (also known as the Chicago Convention), all civil aircraft must be registered with a national aviation authority (NAA) using procedures set by each country. Every country, even those not party to the Chicago Convention, has an NAA whose functions include the registration of civil aircraft. An aircraft can only be registered once, in one jurisdiction, at a time. The NAA allocates a unique alphanumeric string to identify the aircraft, which also indicates the nationality (i.e., country of registration) of the aircraft, and provides a legal document called a Certificate of Registration, one of the documents which must be carried when the aircraft is in operation.