Last year, Aaron Donald and Todd Gurley won offensive and defensive player of the year awards, respectively—just the second time in NFL history two teammates have won the award for the same season.

Through 10 weeks, both appear to be favorites to repeat as winners this season. Gurley leads the league in rushing, rushing touchdowns, yards from scrimmage (139 per game), and combined rushing and receiving touchdowns (with 17 scores in 10 games), while Donald leads the league in sacks (12.5) and quarterback pressures (67) and is the highest-graded defender in the NFL, according to Pro Football Focus. They won NFC offensive and defensive player of the month for October.

Repeating as either offensive or defensive player of the year is rare. Houston Oilers running back Earl Campbell and St. Louis Rams running back Marshall Faulk each won OPOY three years in a row, and Lawrence Taylor and J.J. Watt are the only players to repeat as defensive winners. If Gurley and Donald both repeat, it would put them in extremely rare company as individuals, and it may cement them as the best teammate offensive-defensive pairing in NFL history.

Many teams have had elite players on both sides of the ball, or two players who were the best at their individual positions. But it is exceedingly rare for two teammates to be the best on their side of the ball simultaneously. Donald and Gurley are on the verge of doing it for two years in a row, which would put them in uncharted territory.

Let’s find out just how uncharted that territory is and how Donald and Gurley rank among the best pairings of all time. First, two ground rules for this exercise.

We’re not identifying “elite” or even Hall of Fame players. We’re looking at players who are the best (or have a serious claim as the best) on their side of the ball in a given season.

One must play on offense and one must play on defense. Rice and Montana don’t count. We’re also not counting special teams, so apologies to Devin Hester and Brian Urlacher.

Let’s run through other legendary duos since the merger and figure out their claim to being the best and why they may not stack up.

Terry Bradshaw and Joe Greene, Pittsburgh Steelers (1978)

Why They Stack Up to Donald and Gurley: Bradshaw won MVP in 1978, and the Steel Curtain defense featured one of the best units ever, anchored by Greene.

Why They Don’t: Greene, cornerback Mel Blount, and linebacker Jack Lambert won the DPOY award back-to-back-to-back from 1974 to 1976, but Bradshaw didn’t even lead the league in a given passing category (besides interceptions) until 1977, so it’s difficult to call him the best in season except for 1978. But by then, it was hard to tell who his best defensive teammate was. Greene was merely a Pro Bowler in 1978, and linebacker Jack Ham was an All-Pro. The ’70s Steelers were stronger as a team than as individuals.

Walter Payton and Mike Singletary, Chicago Bears (1984-85)

Why They Stack Up: Payton might be the greatest running back of all time, and the tail end of his prime coincided with the peak Singletary, the finest middle linebacker of his generation. Singletary won Defensive Player of the Year in 1985 (and then again in 1988) while Payton made the 1985 All-Pro team with 2,034 yards from scrimmage. The 1985 Bears went 15-1 in the regular season and hoisted the Lombardi Trophy following perhaps the most dominant playoff run ever, in which they won their three playoff games by a combined score of 91-10 and didn’t give up a point until the Super Bowl.

Why They Don’t: A little-known fact is the Bears weren’t that good for most of the time Walter Payton was good. By the time the Bears defense became legendary, Payton made it back to All-Pro status, but he wasn’t even the best running back, let alone player, in either the 1984 or 1985 season. Eric Dickerson set the all-time record for rushing yards in a season in 1984 (2,105), and in 1985, Marcus Allen was named MVP and led Payton in rushing yards, receiving yards, and touchdowns. Two details hurt Singletary’s case: His teammate, Richard Dent, led the league in sacks that season, and Lawrence Taylor was probably still the best defensive player in football.

Jerry Rice and Ronnie Lott, San Francisco 49ers (1986-87)

Why They Stack Up: The best season from the best defensive back of the decade meets the breakout season from the best receiver ever. In Rice’s second season, he led the league with 1,570 receiving yards and 15 touchdowns and kicked off a stretch of 10 All-Pro seasons in 11 years. Lott, who received the second-most votes for a defensive player picked for the 1980s All-Decade Team after LT, led the league in interceptions, forced three fumbles, and had one finger amputated in 1986.

In 1987, Rice was even better, as he became the first and only receiver to win Offensive Player of the Year (he won again in 1993) as he set a then-NFL record 22 touchdown catches in just 12 games during a strike-shortened season.

Why They Don’t: You can pick a year of Rice’s career at random and argue he was the best offensive player that year, but the same is true for Lawrence Taylor on the other side of the ball. In 1986, Taylor won Defensive Player of the Year and MVP—just the second (and last) time in NFL history a defender has won the award. The next year, Reggie White won Defensive Player of the Year when his league-leading 21 sacks came one sack shy of the then-single-season sack record, and Buffalo’s Bruce Smith finished second with 10 votes. Lott was the best defensive back for years, but he was never quite the best defender.

Thurman Thomas and Bruce Smith, Buffalo Bills (1990)

Why They Stack Up: Smith, the NFL’s all-time sack leader, was an All-Pro in 1987 and 1988 and was named Defensive Player of the Year in 1990. That year, Thomas led the league in yards from scrimmage and was named first-team All-Pro. This was the first of four consecutive seasons that ended in a Super Bowl loss for Buffalo, and both of these players would likely be significantly more famous if Scott Norwood didn’t go wide right (or if the other thousand things that go wrong when a team loses four championships back-to-back-to-back-to-back didn’t happen).

Why They Don’t: That season, Thomas was second to Barry Sanders in rushing yards, Sanders had an astonishing 6.1 yards per touch on the fifth-most touches in the league, Montana took home MVP honors, and Warren Moon won Offensive Player of the Year. Still, Thomas and Smith are one of the few pairs on this list that have multi-year longevity. Thomas was named MVP the following season (1991), but Smith played just five games that season while recording just 1.5 sacks. By the time Smith won Defensive Player of the Year again in 1996, Thomas had been thoroughly surpassed by Sanders and Emmitt Smith.

Jerry Rice and Deion Sanders, San Francisco 49ers (1994)

Why They Stack Up: The best cornerback of all time’s best season and the best receiver of all time leading the league in receiving yards. Sanders had six interceptions and returned them for a league-leading 303 yards (!) and three touchdowns. Meanwhile, Rice led the league with 1,499 receiving yards and added 13 touchdowns as first-team All-Pro.

Why They Don’t: Not even the best pair on the 49ers that year.

Steve Young and Deion Sanders, San Francisco 49ers (1994)

Why They Stack Up: The (scarlet and) gold standard. Young won the MVP, Sanders won Defensive Player of the Year, and the 49ers won the Super Bowl. Young led the league in touchdown passes (35), yards per attempt (8.6), and passer rating (112.8). For perspective, Brett Favre’s 90.7 passer rating was the only other figure above 90 that year.

Why They Don’t: The only knock on this duo is that they played together for only one season. Sanders signed with the Cowboys the next offseason, disrupting what would have unquestionably been the best pairing on this list. Their peak may be greater than anyone else’s, but they can’t match others in longevity.

Deion Sanders and Emmitt Smith, Dallas Cowboys (1996-98)

Why They Stack Up: Smith is the NFL’s all-time leading rusher, and he played with the best cornerback ever for Sanders’s final three All-Pro seasons.

Why They Don’t: Their prime(time)s didn’t align. Smith led the league in rushing three times before Sanders showed up, and he added a fourth rushing title in Sanders’s first season in Dallas—but Deion played in just nine of 16 games that year. Afterward, Sanders was All-Pro each year from 1996 to 1998, but Smith’s production was never the same. He didn’t even make the Pro Bowl in 1996 or 1997 and was 10th in yards from scrimmage in 1998.

Yet there’s a more important reason this isn’t the best teammate duo.

[Puts on helmet, hockey pads, hides in nuclear fallout bunker.]

Emmitt Smith was an All-Time Great, but not as great as “the leading rusher of all time” makes it sound. In 15 seasons, he averaged more than 4.3 yards per attempt just three times. Barry Sanders’s yards per carry (5.0) is significantly higher than Smith’s career yards average (4.2). Smith did not average more than 4.2 yards per carry after turning 27. Minnesota’s current backup running back, Latavius Murray, has a career average of 4.2 yards per attempt. Emmitt Smith was Latavius Murray after turning 27. I will now go into witness protection to avoid Cowboys fans ransacking my home.

Brett Favre and Reggie White, Green Bay Packers (1995-98)

Why They Stack Up: In 1995, a 26-year-old Brett Favre led the league in passing yards and passing touchdowns (one of which went for 99 yards) and won the first of his three consecutive MVPs. Favre failed to win four in a row in 1998 (fatigued voters chose Terrell Davis), but he still led the league in passing yards, and White won Defensive Player of the Year at 37 years old. They also won one Super Bowl together and played in another.

Why They Don’t: While White is considered by many to be the best defensive player in NFL history and Favre retired as the all-time leader in most of the major passing categories, they never truly aligned as the best on their respective sides of the ball. It’s easy to argue for either Deion Sanders or Bruce Smith as being ahead of White during Favre’s peak. In 1996, White had just 8.5 sacks. During White’s DPOY season in 1998, Favre did take a step back from his MVP form and threw six fewer touchdowns and nine more interceptions than he averaged in his three MVP seasons.

Jamal Lewis and Ray Lewis, Baltimore Ravens (2003)

Why They Stack Up: This is the only other pair of teammates in NFL history to win offensive and defensive player of the year in the same season. Ray Lewis’s accomplishments are well publicized, but Jamal’s season has been forgotten. He rushed for 2,066 yards, then the second most in a single season in NFL history (now the third most) and was just 40 yards shy of breaking Dickerson’s record.

Why They Don’t: Jamal Lewis may not have even been the best running back that year. Priest Holmes had almost twice as many touchdowns and LaDainian Tomlinson gained 99 more yards from scrimmage on exactly the same number of touches. Even if we hand it to the Lewises—it was Ray’s second DPOY award in four seasons—Jamal Lewis had less than half the rushing yards the following season and never reached the same heights again, so this match was a one-hit wonder.

Peyton Manning and Dwight Freeney, Indianapolis Colts (2004)

Why They Stack Up: Manning set the modern quarterbacking standard while Freeney led the league in sacks.

Why They Don’t: There’s no question about Manning, but Freeney finished third in Defensive Player of the Year voting behind the winner Ed Reed, the first safety to win the award in two decades, and Steelers linebacker James Farrior. And while Freeney remained elite after his 2004 peak, he never re-entered the conversation as the league’s best defender.

LaDainian Tomlinson and Shawne Merriman, San Diego Chargers (2006)

Why They Stack Up: One of the most dominant running back seasons ever mixed with Merriman leading the league with 17 sacks in just 12 games.

Why They Don’t: There’s zero question Tomlinson was the best player on offense in 2006 (if you need a reminder: 2,323 yards and 31 touchdowns from scrimmage) but Merriman missed four games for violating the NFL’s banned-substance policy. Defensive Player of the Year went to defensive end Jason Taylor. Merriman got just six votes but was on pace to beat Michael Strahan’s single-season sack record. Merriman had just 18.5 sacks in the next six years, so his fall was steep.

Peyton Manning and Bob Sanders, Indianapolis Colts (2007)

Why They Stack Up: Manning was the best quarterback of the young century (the only five-time MVP in history, winning in 2003, 2004, 2008, 2009, and 2013) and Sanders was named the 2007 Defensive Player of the Year.

Why They Don’t: The 2007 season was the one year of his prime Manning was definitely not the best quarterback: Tom Brady rewrote the record book as the Patriots went 18-1. Sanders wouldn’t play more than six games in a season again afterward.

Aaron Rodgers and Charles Woodson, Green Bay Packers (2009-11)

Why They Stack Up: Woodson led the league in interceptions in 2009 and 2011 and was named Defensive Player of the Year in 2009 and a first-team All-Pro in 2009 and 2011, while Rodgers was named MVP in 2011 with 45 touchdowns to just six interceptions, 4,643 yards, and a 14-1 record.

Why They Don’t: Rodgers wasn’t quite the best quarterback in the league yet in 2009 (Manning was MVP, Drew Brees led the league in passing touchdowns, and those two met in the Super Bowl) and running back Chris Johnson rushed for 2,000 yards. When Rodgers took the QB title belt in 2011 by winning the Super Bowl that February and then being named MVP the subsequent season, Woodson was still elite, but he wasn’t the best defender in football. Baltimore’s Terrell Suggs was named the Defensive Player of the Year, and New York’s Darrelle Revis had usurped Woodson as the league’s best cornerback.

Peyton Manning and Von Miller, Denver Broncos (2012-15)

Why They Stack Up: Manning submitted the best season in the history of quarterbacking in 2013 just as Miller was blossoming into the league’s best edge rusher.

Why They Don’t: J.J. Watt.

Manning’s best season coincided with Miller’s six-game suspension for attempting to cheat the NFL’s drug testing policy. One year earlier, Miller had 18.5 sacks, but J.J. Watt had a near-unanimous DPOY award. Watt also won the award each of the next two seasons, and by the time Miller may have claimed his spot in the 2015 season by winning Super Bowl MVP, Manning’s noodle arm had rendered him one of the worst quarterbacks in the league.

Todd Gurley and Aaron Donald, Los Angeles Rams (2017-18)

Why They’re the Best Pairing in NFL History: If the season ended today, they would be back-to-back winners for OPOY and DPOY. As the above list shows, nothing like that has happened before. Donald is head and shoulders the best defensive player in football, and Gurley is clearly the league’s best non-QB offensive player through 10 weeks, and it’s not outside the realm of possibility that Gurley wins MVP this season.

Why They May Not Be: They haven’t won yet. Even if they do, the biggest question is whether Gurley can be considered the best offensive player in a quarterback-dominated era. As Drew Brees and Patrick Mahomes II lead their teams to soaring offensive heights, it’s fair to wonder how impactful Gurley is by comparison (and how many other running backs could succeed in Gurley’s place). Yet that’s true for many of the offensive and defensive players in NFL history—including many on the above list—and none of them have been able to do what Donald and Gurley appear to be on the verge of doing.