As originally designed, the IND train identification scheme was based on three things: the Manhattan trunk line served (8th Avenue or 6th Avenue), the northern branch line served (Washington Heights, Grand Concourse/Bronx, or Queens Boulevard), and the service level (Express or Local). The 8th Avenue routes were A, C, and E, while the 6th Avenue routes were B, D, and F. The A and B served Washington Heights, the C and D served the Grand Concourse, and the E and F served Queens Boulevard via the 53rd Street Tunnel. A single letter indicated express service, while a double letter indicated local service. In addition, G was used for Brooklyn-Queens "Crosstown" service, and H was used for any service on the extended Fulton Street (Brooklyn) line that did not originate in Manhattan.