The New York City Subway's B Division consists of the lines that operate with lettered services (A, B, C, D, E, F, G, J, L, M, N, Q, R, W, and Z), as well as the Franklin Avenue and Rockaway Park Shuttles.

With the Dual Contracts, signed in 1913, the BRT acquired extensions outward into Queens, as well as through Lower and Midtown Manhattan.

Since the original IRT tunnels were smaller, it has remained a separate division to this day.

IND services were labeled on maps and signs starting with the opening of the first line in 1932.

Only Southern Division routes (1–4 or N–T) were labeled on maps, but all services except remnants of the old els were assigned letters:[5][6] * Unofficially signed as "M", or sometimes "S".