I remember a few years back someone caused a bit of a kerfuffle on a United board by referring to business class as J. One or two posters insisted that it always be referred to as C on the United board.

That's probably the biggest difference in fare buckets I've noticed in my years of flying as an AA Plat in some years and a UA 1K in others (J vs. C). The rest of them were fairly consistent across (most) airlines, with slight variations on the sequencing of discounted-coach fares as you got close to the rock-bottom fares. My cheapie AA fares back in the day were usually L. My cheapie United fares were often S.

Q was an oddball that got used for lots of things on UA. I've had cheap restricted Q's, fully-refundable Q's, and Q-UP's. It seemed to me that it was purposely vague so they could use it for special purposes...for example, the Q-UP's were used to compete against Midwest Express: otherwise, the -UP's were usually up in the H, B, or Y range.

It would be fun to really hear from an insider how they came up with the codes and what exactly each one meant originally and how that's evolved. Total airline nerdery of course...