Originally, bus routes were given franchise numbers by New York City. These franchise numbers used borough prefixes (M for Manhattan). The utilization of these franchises was different by different operators.
Fifth Avenue Coach did not use the franchise numbers, and put its routes together in such a way that they actually did not correspond to the franchised routes (a single Fifth Avenue route might be pieced together from different franchises). Therefore there is no easy correspondence between Fifth Avenue routes and franchise numbers.
New York City Omnibus Co. also used its own proprietary route numbers, but the routes mostly did correspond to franchised routes, albeit with different route numbers.
All other operators tended to use the franchise numbers assigned by the City to their routes.
When Manhattan & Bronx Surface Transit Operating Authority took over Fifth Avenue, NYCO, and Surface Transit, Inc. routes, a uniform M-prefixing was done to all routes (except for former FACo 15, which operated mostly in Queens, and FACo 16, which operated entirely in Queens, both of which got Q-prefixed numbers). This meant that NYCO Route 1, whose franchise number was actually M25, became M1, and created some duplications. As a result, several routes got renumbered (for example, New York City Transit Authority Rte. M1 became M22). In addition, because of one-waying of avenues, sometimes different routes became combined (for example, FACo. Rte. 1 and NYCO Rte. 1 were both combined in MaBSTOA's new M1).
These tables (restricted to routes that operated in Manhattan) show the correspondence between old route numbers and new ones.
Colors for current routes are as used on current MTA bus maps.
See alsoEditBus consolidation in New York City