If you’ve ridden the New York City subway, you know that the different trains are often designated by letters of the alphabet (or numbers, but we’re not talking about those).

If you’ve ridden the New York City subway a lot, you know that some letters are missing. And it’s not the tail end of the alphabet, either. There’s an A, B and C train and there’s also a Z train. But there's no H train or Y train. There are several other skipped letters, too.

The explanation for the missing letters falls (mostly) into two categories.

Discontinued

H, K, T, V and W are all former trains that have been discontinued. In fact, an in-depth look at the history of the Metropolitan Transportation Authority (MTA) reveals regular recycling of letters as trains go in and out of use. But let’s look at each of the current letters getting snubbed.

H

Technically, the H train is still in use. It is the internal name for the Rockaway Park shuttle in Queens that connects with the A train to provide service to the western edge of the peninsula. After a few years without any designation on the map, the shuttle became known as the HH train in 1962. Over the next several decades, it was known as the E and then the CC train. In the 1980s, the MTA did away with double letter designation and the shuttle earned its H name. Eventually, however, all shuttles started bearing the grey S designation on maps. Since this would create internal confusion, the MTA continues to call the Rockaway Park shuttle the H train for their own purposes.

H made a public resurgence as the name of a free shuttle that took over service for a suspended portion of the A train in the aftermath of Hurricane Sandy in 2012. The letter was available as the storm had also knocked out service on the Rockaway Park Shuttle. When both the shuttle and the A train came back online by the summer of 2013, H slipped back into hiding.

K

The K train used to run on the same Eighth Avenue track that currently sports the A, C, E. It was the name given to the former AA train when the use of double letters was discontinued in 1985. The K train was essentially the opposite of the C. Both ran locally between 168th Street and the World Trade Center, but while C train service covered rush hours, the K took on midday, nights, and weekends. The K last ran on December 10, 1988; the next day, C train service was expanded to include all hours.

T

T (and TT) used to be the name of what is now known as the D train. It was absorbed by the B train in the 1960s, which was replaced by the W train and ultimately the D train in the early 2000s. If you miss the T train, never fear: It is the future name of the Second Avenue Subway, which is expected to open its first phase in December 2016.

V

Longtime New Yorkers may remember both the introduction and the dissolution of the short-lived V train. It debuted in December 2001 to replace a rerouted F train, traveling between the Lower East Side in Manhattan and Queens. Unfortunately, limited service patterns—the V only ran weekdays from approximately 6:30 a.m. to midnight—and budget cuts doomed the V train. In June 2010, it was merged with the M train and dropped the V designation.

W

Another short-lived, late-alphabet train, W debuted in 2001 to account for construction on the Manhattan Bridge tracks that disrupted the B train. During nearly nine years in use, its service area changed several times, going all the way to Coney Island to start, then eventually losing the Brooklyn borough altogether. In Manhattan, W traveled along the Broadway line that we now think of as the N, Q, R train, eventually getting absorbed into those lines in 2010 as a result of financial shortcomings.

Potential Confusion

The letters I and O were never used for trains because of their visual similarities to the numbers 0 and 1 and the use of both alphabetical and numerical designations in the New York Subway system.

U and Y were eliminated from consideration because they are homonyms with actual words—namely, “you” and “why.” Apparently, the founders of what is now the MTA did not find the potential for “Who’s on First”-style confusion to be nearly as humorous as Abbott and Costello did.

The Problem with P

That leaves all the letters accounted for except P. There are several stories of almost-P trains. It was the intended name for the final leg of the Culver service (now the F line) into Manhattan that was scrapped in favor of a shuttle (labeled S as all shuttles are) at the last minute. It was also the name given to a specialty train designed to work around the 1992 potential Amtrak strike. Congress intervened at the last moment and no such emergency service was necessary. The P then stood for the incapacitated Penn Station. However, neither of these instances explain why it was never used for any new trains between the creation of the Culver shuttle and the 1992 near-strike. And so it is fun, if not entirely likely, to consider the theory that the P homonym was simply too immature for polite commuter conversation.