A section of a poster promoting the 1948 film “Sorry, Wrong Number.” (Paramount Pictures)

There is a theoretical world out there in which every phone number starts with 555. This is such a well-known feature of our culture by now that the use of “555” to start phone numbers in television shows and movies has itself been an inside joke multiple times, in the movie “The Adventures of Ford Fairlane,” for example, and on “Family Guy.” Why 555 serves this purpose is equally well known: Giving out numbers using other sets of numbers runs the risk of random people suddenly being called by people looking to speak with someone they saw on TV. (Ask anyone whose number is 867-5309.)

But then you come across a tweet that illustrates an interesting twist to this idea.

Theoretically, there is a unified universe that relies on 555 for all of its phone numbers — meaning that, in this universe, the number that was once affiliated with Homer Simpson’s snowplowing service is now the number for the fictional Southpaw Regional Wrestling.

This conundrum — a boundless universe constrained by limited phone numbers — was itself lampooned in the movie “Last Action Hero.”

Danny Madigan: I will bet that everyone has a 555 number.

Jack Slater: So?

Madigan: So? There can only be 9,999 numbers that start with 555! How many people live in L.A.?

Jack Slater: Eight or nine million.

Madigan: Aha!

Jack Slater: That is why we have area codes.

As it turns out, the former governor of California is sort of right and his companion wrong — and not only about there being 9,999 numbers. (Including 555-0000 and 555-9999, there are 10,000.)

Scouring movie and television script sites, I put together a directory of as many 555 numbers as I could find. This is an incomplete list; additions are very welcome. Below, the first interactive directory of the 555 exchange.

Philip Bump/The Washington Post

Many of these numbers are actually “KL5″ numbers — that is, Klondike-5 numbers. What we traditionally have thought of as the first three numbers of a phone number — especially those of us who lived in a time when area codes weren’t commonly included when numbers were shared — was once the indicator of the local exchange that number used. For example, in a 1932 Manhattan phone directory, you can see the numbers listed by exchange. CH-elsea. ST-uyvesant. This was before direct dial, so there was no need to figure out what CH or ST converted to as numbers on your rotary dial; you’d simply ask the operator to connect you. Once people dialed directly, the first two letters became the first two numbers of the exchange. In that 1932 phone book, the number for the store Bergdorf Goodman, located near the Plaza Hotel, is given as Plaza 3-7300. That translates to 753-7300, which is still the store’s phone number today.

KL — as in Klondike — translated to “55.” And so: 555. How this tradition of using that exchange for fictional numbers began isn’t really clear, it seems. (Mental Floss looked at this a while ago.) You may not be aware, though, that the 555 exchange isn’t actually reserved exclusively for fictional numbers. Only numbers in the 555-0100 to 555-0199 range are actually reserved. Which is one reason why, when I grouped the numbers I found, there were far more 555 numbers in the 0100 region than anywhere else.

Okay, but back to Arnold being right.

Of the numbers that I identified, a third were in use by more than one different fictional user — like the Mr. Plow/wrestling conflict above. Ernie Douglas from “Cable Guy” and Paul Buchanan’s mother from “Mad About You” are both at 555-4329, for example. But the Cigarette Smoking Man from the “X-Files” is not the same as Michael Rivkin from “NCIS” — because the former’s office is 202-555-0130 and the latter’s home is in the 310 area code. When you account for area codes, the number of duplicates falls to only about a fifth.

Some of those conflicts are internal to television shows. 555-8383, for example, is both Jerry Seinfeld’s car phone number (in the episode where his car is stolen) and the number George Costanza gives for Vandelay Industries — meaning that it’s Jerry’s apartment phone number, too. But Jerry’s number is also given as 555-2390 when Elaine Benes wants to avoid talking to the man at the pen store.

In other cases, it seems only right that a number should be consistent between two fictional universes. On “Seinfeld,” 555-LOVE is an escort service. It’s also the number used in “Do the Right Thing” — set in a very different part of New York City — for Mister Señor Love Daddy’s cash giveaway during his radio show. One can easily imagine a scenario in which that number never really changed hands.

The most popular number I found? 555-0199. It’s the Syracuse Police Department on “Homeland,” with an 800 area code. It was also the Pacific Bay Police Department on “No Ordinary Family.” It is the number for a producer on “60 Minutes,” for a “how am I driving” call center, for Dr. Becker’s office on “Becker,” and, most prominently, for one Fox Mulder on “The X-Files.”

One can envision a universe in which all of those numbers correctly point to the same receiver as well. You simply need to be willing to suspend your disbelief.