Call letters, or a call sign, was a radio identifier used for identifying a starship. This identifier was an alphanumeric sequence. Ships carried in their library computer's database a listing of the call letters of most registered ships and planets. Ships operating in neutral space that used call letters weren't always known by name. When a ship was in distress, it was customary for that ship to include its call letters in the distress call. When a ship was acting friendly, it was customary for it to transmit its call sign. An official organization assigned a unique call sign to each ship.