TThis item has been corrected.

At first glance, there is logic to New York City’s numbered subway lines: 1, 2, 3, 4, 5, 6, 7.

The letters, however, are a little less straightforward: A, B, C, D, E, F, G, J, L, M, N, Q, R, S, S, Z. What happened to the H? Or the O? And why in the world there are three S trains—shuttles going between Grand Central and Times Square in Manhattan, between Prospect Park and Franklin Avenue in Brooklyn, and to Rockaway Park—when there could easily be an S and, say, a K?

To understand how this came to be, we need to go back 111 years, when the subway opened in Manhattan on Oct. 27, 1904.

1904

At the time, the subway, known as the Interborough Rapid Transit (IRT) comprised of 28 stations in Manhattan. Eventually, the Brooklyn Rapid Transit (BRT), which in a few years became the Brooklyn-Manhattan Rapid Transportation (BMT) joined the IRT. Initially, the BMT lines were marked with numbers 1 to 16 and, as reported by Joseph D. Korman, a transportation enthusiast and web master who has compiled a history of the Metropolitan Transportation Authority (MTA).

Courtesy of New York Transit Museum IRT map, 1904

The IRT was known as the “H system” since it ran along parallel north-south routes, on Manhattan’s seventh avenue on the west and Lexington Avenue on the east, connected in the middle through the 42nd street shuttle (between Grand Central Station and Times Square). However, the system’s lines didn’t adopt numbers or letters until 1949, when they started being marked with numbers from 1 to 9. By the 1960s, the 8 and 9 lines had been discontinued, and the system counted a total of four shuttles, all named “shuttle,” though the 9 was reintroduced between 1988 and 2005 to indicate the alternate stops on the 1 line.

1932-1950s

The IND system, opened in 1932, was the first to be owned and run by New York City. Through the years, it expanded to include eight lines—A, B, C, D, E, F, G, H—and two special routes—the S, to the aqueduct race track, and the ”Train to the Plane,” the JFK Express (opened in 1978).

Courtesy of New York Transit Museum 1938 map of IND lines.

In 1940, the city also bought the BMT and IRT systems, becoming the operator of the full subway system. In 1953, the New York City Transportation Authority (part of the MTA) was established to run the transportation system.

Map courtesy of mta.info 1939 IRT map.

When both the IND and the BMT were taken over by the Transportation Authority, the BMT’s lines were named J, K, L, M, N, Q, R, T, and two S shuttles. Initially, double letters were employed to mark a local train (i.e. JJ or KK), and the Q line was marked as QB (going over the Manhattan bridge) and QT (via the tunnel), though both distinctions were eventually dropped. Through the history of New York City subway, lines were named using two letters to mark different routes as well as different service—as was the case of the NX, a special express train to Astoria operating during rush hours.

Aside from the lettered lines, the BMT still ran the lines lines 10, 11, 14, 15, 16, which were elevated.

Courtesy of New York Transit Museum 1948 BMT map.

1967-1970s

On Thanksgiving weekend in 1967, the “Chrystie Street Connection” was opened joining IND and BMT lines, which made some of the numbered lines obsolete. By then the 12 and 13 lines had been discontinued and now the other numbers from 10 to 16 were too.

The system now included nine shuttles—four from the IRT, three from the BMT, two from the IND. The seven IRT and BMT shuttles, all called S (or SS, as they were local), served Dyre Avenue, Grand Central, South Ferry, the Lenox Terminal, Prospect Park, Ditmas Avenue, and Metropolitan Avenue. The IND shuttle, called H (HH), went to Rockaway Park and Far Rockaway.

Around this time, the lines were color-coded, and all the different systems began to appear on the same map. In 1972, the MTA introduced an innovative representation of the subway map, by Massimo Vignelli:

Courtesy of New York Transit Museum Massimo Vignelli’s map, 1972

The colors were not the same as those used today. For example, the 4 was purple (now green) and a blue color was used to mark the defunct K.

1980-2000s

In the years that followed the merging of the systems, and in particular during the 1980s, some letters appeared and others disappeared. The reduction of service carried on until the early 2000s, due to budget cuts as well as redundancy in the lines, though according to the MTA the ridership grew to over 2 billion passengers a year. The H was added, then repealed. In 2001, both the W and V were added and then eliminated, replaced by the Q (extended to cover the W route in 2008) and M (for the V in 2010). Also, the shuttles were reduced to two—both S.

Courtesy of New York Transit Museum Subway map, 1997

2015

Some letters have never been employed, and never will “for more or less obvious reasons,” Korman told Quartz—I and O, for instance, look too similar to 1 and 0. Others are waiting to be brought back—like the T, which will identify the long-awaited Second Avenue line, scheduled to open at the end of 2016, which has been in the works since 1919 (and will be called Q for till the third segment is completed).

Many of the lines that have disappeared aren’t gone at all, and have been replaced by expansions on other lines:

Former line Replacement H A, C K C T B V M W N, Q V M 8 N, Q 9 7 10 M 11 M 14 E 15 J 16 L

None of this, however, explains why there are still three S trains. But I guess, it’s better than nine Ss.

Correction: there are three S trains.