The New York City Subway is a heavy-rail public transit system serving four of the five boroughs of New York City. The present New York City Subway system inherited the systems of the Interborough Rapid Transit Company (IRT), Brooklyn–Manhattan Transit Corporation (BMT), and the Independent Subway System (IND). New York City has owned the IND since its inception; the BMT and IRT were taken over by the city in 1940. The former IRT system is now known as the A Division, while the B Division is the combined former BMT and IND systems. In the New York City Subway nomenclature, a "line" refers to the physical trackage used by trains that are used by numbered or lettered "services". Today, the division or company names are not used publicly, while the line names may occasionally be used. The services that run on certain lines change periodically, but the lines refer to static trackage. Nomenclature In the nomenclature of the subway, the terms "line" and "service" are not interchangeable with each other. While in popular usage the word "line" is often used synonymously with "service" (even sometimes on the website of the MTA), this list will use the formal usage of the term "line." A line is the physical structure and tracks that trains run over. Each section of the system is assigned a unique line name that begins with its original division (IRT, BMT or IND). For example, the line under Eighth Avenue is the IND Eighth Avenue Line. Some lines have changed names (and even divisions), but this happens relatively infrequently. By contrast, a service refers to the route that a train takes across the various lines. A service can operate along several lines and even along different divisions. For example, the service operates along the IND Queens Boulevard Line as well as the BMT Broadway Line and the BMT Fourth Avenue Line. Each service is also assigned a color. Since 1979, each service's color corresponds to the line it primarily uses in Midtown Manhattan—defined as the trunk line. There are three exceptions: the IND Crosstown Line, which does not carry services to Manhattan, is colored ; the BMT Nassau Street Line, which runs only through Lower Manhattan, is colored ; and all shuttles are colored . The list of trunk lines and colors is shown in the table below. Line listing There are currently rail lines. The Archer Avenue