This article is from the The Simpsons FAQ, by Ondre Lombard wakkojr@nospam.wcat.com and Andrew Gill superluser@mail.rit.edu with numerous contributions by others.
That's probably the p-code. Production codes are formed in the
following way:
[season code] [Letter] [episode number]
- The seasons run from 7 to 5 (so far) and AAB, with the airing order
going 7 8 9 1 2 3 4 5 AAB.
- The letter is either an F or a G.
- G indicates a special series (not really, but that's the best
explanation that I can give) Season 7G was Season 1, and episodes
3G__ were special leftovers from season 7.
- F is the typical letter used in an episode. It doesn't officially
stand for anything. (My conjecture--Fox)
- The episode number is the part from the series from whence it came.
There are typically 24 episodes per series, with two coming from the
previous production run.
- Certain p-codes are common enough to be worthy of inclusion: The
Halloween Specials are always an episode from 01-04 (usually 04).
Other specials, like the 138th Episode Spectacular, are assigned
special numbers, like 3F31.
- Usually, a few episodes are produced that don't air until the next
season. These are usually episodes 22-24. So the seasons are not
always the same as their p-code indicates.
 
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