The alphabet game involves trying to find a written instance of each letter of the alphabet while traveling by car. Sometimes the rules require that the letters appear only on license plates (so it's also known as "the license plate game"); sometimes they can also appear on any sign or other textual context.
The blog Oddly Specific provides a photo of a road sign in Wyoming that helps out travelers who are stuck in the game.
X is easy (all those EXITs). J is the sticking point.
That's incredibly useful. But it would be even more useful if it had a "J" on it. I always found those much harder than Y, or even X (which is easy on a freeway).
(and I swear I didn't see the prior comment before I posted...)