See a strange area code on caller ID? The website of NANPA (the North America Numbering Plan Administration) is the place to search for information about area codes, find up-to-date area code maps for North America, and up-to-date info on recent and upcoming area code splits, and on the dialing rules in each area. Where can I use 10-digit dialing? Where must I dial all 10 digits?

Remarkably, 13 states -- mostly in the Mountain West and in New England -- and DC still are managing with a single area code.

And here's a site with the history of area code numbering. Oklahoma became a two-area code state when 918 was created in 1953 (mandatory in 1954). The rationale behind the original numbering -- most populous areas got the fewest pulses -- New York had the most favorable area code, then Chicago and Los Angeles.