Tom Brady and Peyton Manning are clearly future Hall of Fame quarterbacks, and much of their legend stems from their numerous contests – born out of the good fortune for the two to play for perennially dominant teams in the same conference for over a decade – that gained such iconic status that they gained Super Bowl-esque Roman numeral status. That series is now coming to what is surely its final chapter, in the form of Brady-Manning XVII – and, as is surely only appropriate, it’s an AFC Championship Game. The end of this era provides an opportunity to look back and pace it in its historic context.

When we trawl through the waxwork museum of NFL legends – be it literally amongst the busts in Canton or metaphorically through nostalgic conversations and viewing historic footage – our thoughts are invariably shaped by the nature of the game at that time, and how it has changed since. In particular, we might look at all those legendary running backs and look back (perhaps fondly) at the days where teams would run first, run second, and pass only in the act of desperation or deception.

But when you’re talking about Tom Brady and Peyton Manning, you are talking about a new era. A new century, a new millennium – both were technically drafted in the 20th century (Brady’s draft year, 2000, still counts because there is no year 0) but they made their reputations as the twin titans of the NFL in the early 21st century, and in many ways came to define that era.

What will that era look like a generation from now? Quite different from the one before that, we know that much. Teams started to throw more often, that much is obvious (and ongoing, with a continuing drip-feed of passing-friendly rule changes throughout the century to date encouraging an aerial attack). What is less obvious, and perhaps more interesting, is the very shape of the offense.

This is something that has become very easily lost in the shuffle of “today’s college quarterbacks can’t run a real pro offense” – a stick used to beat even top-of-the-draft prospects like Marcus Mariota in recent years, and never mind that when the Philadelphia Eagles installed Mariota’s “college offense” in the NFL they went from a 4-12 embarrassment to a 10-6 divisional champion in one season. What went seemingly almost unnoticed is that many an NFL offense had already met collegiate concepts at least halfway.

The stereotype of a college offense is one where the quarterback doesn’t actually do much with his arm, just run a bit and throw a few quick throws. Leaving the running aside for a moment, what do many NFL offenses look like now? An awful lot of them are filled with plays with the quarterback taking the snap and then quickly flinging it to a receiver near the line of scrimmage – and, crucially, that includes not only those built around lower-tier noodle-armed signal-callers, but the record-breaking offenses of Brady and Manning.

The pro game was at this first, to an extent; the West Coast offense of the 1980s flipped the script of “use the run to set up the pass” by being loaded with short passing plays that effectively worked like running plays except for spreading defenders out of the tackle box rather than luring them into it, and to this day that remains the foundation of many – most? – NFL offenses. They might uncork a few more bombs these days, but that’s at least in part because there’s a pretty good chance of gaining a bunch of yards on a “what was that for?” pass interference flag even if the throw isn’t quite right. (The ability of today’s wide receivers to combine modern sophisticated gloves with immense athleticism to make previously unfathomable catches hasn’t hurt.)

Speaking of the pass interference rule changes, it is an underappreciated part of the Brady-Manning legacy that the 2004 change (though technically a clarification of one in 1994, the removal of contradictory language meant that the actual in-game impact was truly the equivalent of a new rule) came about as an almost direct response to the previous season’s Brady-Manning IV, the first playoff meeting between the two. In that game – also the first Brady-Manning game at the then-new Gillette Stadium – Bill Belichick nullified Manning’s Indianapolis Colts with rough-and-tumble coverage from defensive backs who correctly figured that the officials would be loathe to get their flags out in a playoff game, leaning on the ambiguities of the 1994 wording. Those ambiguities went, the flags started to fly, and the mass adoption of the spread in the pro game as a way to take advantage of a rule set that all but begged teams to air it out effectively began here – a new era in the NFL.

Not that the spread offense was a new concept. In fact, it has its origins in the middle of the 20th century; in the 1950s, Leo “Dutch” Meyer, TCU coach at the time, wrote the book “Spread Formation Football” about his short-pass-heavy offense that (unthinkably for the time) didn’t use a blocking tight end, and a decade later, high school coach Glenn “Tiger” Ellison wrote the 1965 book “Run-and-Shoot Football: The Offense of the Future” – a grandiose claim that turned out to be more right than wrong. Ellison’s offense in its original form was built around both the quarterback and his receivers making pre-snap reads to anticipate where to go, but the principle from that scheme that really did form the basis of the “offense of the future” was loading up on outside receivers.

By the time Brady and Manning were playing in college, the spread was already starting to become more common. By 2004, it was common, routine even, for collegiate teams to use pass-heavy offenses with multiple wide receivers on the field – and for quarterbacks, sometimes without even a single runner next to them in the backfield, to take their snaps from the shotgun position several yards behind the line of scrimmage, rather than under center.

If that sounds like something you now see on Sundays, that’s because you do. Pro Football Focus data from Peyton Manning’s record-breaking 2013 season show that he took the ball to pass from the shotgun on more than 500 snaps that year. That’s almost eight times per quarter, and even that is discounting his snaps from the “pistol,” closer to but not directly under the center. Furthermore, Peyton released the ball in an average of less than 2.4 seconds on those plays, despite having a standout deep threat in Demaryius Thomas as his top receiver. That same year, Carson Palmer led the league in the percentage of passes in which he dropped back from under center… at less than 43%. That is a seismic change from even ten years earlier, where multiple teams called zero shotgun snaps all year according to game tracking data from Football Outsiders, even though the effectiveness of a spread offense at the professional level had already been shown by the St. Louis Rams and their “Greatest Show on Turf” offense. (Of course, it was at that team’s expense that Brady claimed his first Super Bowl – and that, too, was with the help of a Patriots defense that got physical in coverage in a way that would draw a flotilla of flags today.)

That’s the direction the NFL has gone. Complaining about collegiate quarterbacks “not taking snaps from under center” and “constantly throwing short passes” sounds almost churlish, archaic even, when one considers that Peyton Manning threw his way into the record books not taking snaps from under center and constantly throwing short passes. The fact that he did so in his mid-30s – and two years after career-threatening neck surgery! – is not only a remarkable monument to his qualities as a player and competitor, but an illustration of how spread offense principles aren’t just a way for a cocky young athlete who can fire off a hundred quick outs in between scrambles to become a campus superstar.

Of course, many a player has gone into the NFL after such a college career and become a notorious bust, even when their offenses have been tailored to them to a greater or lesser extent. This is where the individual attributes of Brady and Manning have come in, of course – and, in particular, their ability to read defenses. That’s the other reason why both could produce their very own 5,000-yard seasons at an age where most of their peers are watching their kids do so on a video game. From the same shotgun position still too often seen as the crutch of a mentally unready quarterback, Brady and Manning could survey the field in front of them and make pre-snap adjustments more effectively than they could from under center by simple virtue of being in a better position to read the defense, and end up as devastating playmakers without needing to sprint out of the tackle box, or indeed move very much at all.

In years to come, the NFL will change, just as surely as it has changed in the past, but this has been the era of the pass-happy shotgun spread in the pro game as much as it was in the collegiate one. Whether the next move is a “back to the future” reversion to old-school dropback-and-handoff offenses as a counter to increasingly spread-oriented defenses built for speed and not for power, or every team in the league running a Chip Kelly-esque no-huddle read-option spread, or something else currently unforeseen and unforeseeable, is irrelevant. The NFL of the start of the 21st century will be remembered decades from now as a particular model of how this sport is played, and Brady and Manning will be remembered as its greatest exponents.