Pre-1990: Each conference sent three division winners, plus two wildcards, to the playoffs. Weekend #1 ALL THREE division winners got a bye. The two wildcard teams would play each other. The winner of that game played against the #1 team in the conference, while the other two rested teams played against each other.
I'm not sure if this was always the case all the way back to 1970, or if maybe it started out with just one wildcard team per conference? That seems reasonable.
1990-2001: An extra wildcard team was added. Division winners with the top 2 records received byes, while the division winner with the worst record and all three wildcard teams played off in Week 1 of the playoffs. The weakest division winner was always made the #3 seed, and the other teams were seeded in order. Higher-seeded winner got team #2 in the next round; lower-seeded winner played against #1.
2002-now: Still 6 teams per conference, but now 4 divisions per conference. It's still an important change though.
Now, we see the #5 seed as being (usually) a very strong team, whereas #4 is usually weaker by comparison. Take this year for example in the AFC. Tennessee was a #5 (12-4), Baltimore was #4 (10-6 despite playing 6 games against relatively weak division rivals Cleveland, Cincinnati and Pittsburgh). Most years now, the way things fall, the #4 team in at least one of the conferences is going to have it's hands full with a real, quality #5. Mathematically, the odds of the four best teams in a conference all coming from different divisions is unlikely.**
In prior years though, it was the #4 team that was usually very strong...and the #3 team that was a "weak" division winner. But #3 and #4 would NEVER meet in the playoffs (unless they both made it to the conference final...it's the only way possible).
It changes the entire dynamic of the playoffs having this new system!
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** - If my math is correct, the odds of the four best teams in a conference all coming from different divisions is about 9%.
The first team can come from any division.
The second team has a 75% chance of not being in the same division (3 divisions out of 4).
The third team has a 50% chance of not being in one of
those two divisions.
The last team has a 25% chance of being in the final division.
75% x 50% x 25% = 9.4%
Of course teams in the same division play each other twice a year, instead of once or not at all...which increases the chances of a non-division winner having a worse record than would otherwise be the case because they may have had to play the #1 team twice... etc etc.
Ok, that's more than you asked for and my head hurts now...
