With the Broncos and the Seahawks — last year’s Super Bowl contestants — hosting playoffs games this weekend, the N.F.L. may seem like a closed club, with the same few teams winning every year. Actually, that has never been less true than now.

Once upon a time, it was common for teams to advance to the Super Bowl two years in a row. It has happened 18 times since the Super Bowl debuted after the 1966 season — beginning with the first two, won by the Green Bay Packers. And through the 1990s, there had never been a stretch of more than three years without at least one team making consecutive appearances.

But for the last nine years (since the 2003 and ’04 New England Patriots), no team has repeated as conference champion. If neither the Broncos nor the Seahawks win this weekend and again the following week, that will make it 10 straight seasons.

Of course, some teams, like the Patriots and the Steelers, could remain strong over an extended period without making it to the title game in consecutive years. If a team, say, made it to the Super Bowl every other year during a decade, wouldn’t that constitute a “dynasty”? Fair point — but the evidence is that even by that standard, N.F.L. dynasties are on the decline.