Originally, back in the middle 1950's when direct dialing of long distance calls first became possible, the idea was to assign area codes with the 'shortest' dialing time required to the larger cities.
Touch tone dialing was very rare. Most dialed calls were with 'rotary' dials. Area codes like 212, 213, 312 and 313 took very little time to dial (while waiting for the dial to return to normal) as opposed, for example, to 809, 908, 709, etc ...
So the 'quickest to dial' area codes were assigned to the places which would probably receive the most direct dialed calls, i.e. New York City got 212, Chicago got 312, Los Angeles got 213, etc ... Washington, DC got 202, which is a little longer to dial than 212, but much shorter than others.
In order of size and estimated amount of telephone traffic, the numbers got larger: San Francisco got 415, which is sort of in the middle, and Miami got 305, etc. At the other end of the spectrum came places like Hawaii (it only got statehood as of 1959) with 808, Puerto Rico with 809, Newfoundland with 709, etc.
The original (and still in use until about 1993) plan is that area codes have a certain construction to the numbers:
The first digit will be 2 through 9. The second digit will always be 0 or 1. The third digit will be 1 through 9.
Three digit numbers with two zeros will be special codes, ie. 700, 800 or 900. Three digit numbers with two ones are for special local codes, i.e. 411 for local directory assistance, 611 for repairs, etc.
Three digit codes ending in '10', i.e. 410, 510, 610, 710, 810, 910 were 'area codes' for the AT&T (and later on Western Union) TWX network. This rule has been mostly abolished, however 610 is still Canadian TWX, and 910 is still used by Western Union TWX. Gradually the '10' codes are being converted to regular area codes.
We are running out of possible combinations of numbers using the above rules, and in 1995 southern Alabama will split off area code 205 and become 334.
I hope this gives you a basic idea. There were other rules at one time such as not having an area code with zero in the second digit in the same state as a code with one in the second digit, etc .. but after the initial assignment of numbers back almost forty years ago, some of those rules were dropped when it became apparent they were not flexible enough. One rule that still applies is that no area code crosses a US state line, although in Canada some provinces and/or territories share codes.
Patrick Townson TELECOM Digest Moderator