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For the past three or four years, one of the things we have known about baseball—just known, without question or reservation—is that David Ross was a vital part of Jon Lester’s success. Recall that Ross (who, we all knew, was an elite pitch framer) stepped in as the Red Sox’s primary catcher when Jarrod Saltalamacchia (in addition to being a terrible defender) got exposed by good pitching during Boston’s 2013 World Series run, and then became Lester’s personal catcher beginning in the spring of 2014.

Lester had a 2.02 ERA and held opponents to a .581 OPS when working with Ross that season, before being traded away from his new partner at the trade deadline. He still pitched well for the A’s, who paired him up with Derek Norris, but his opponents' OPS rose by about 50 points, and then the 2014 Wild Card Game happened.

It was clear that Lester was, to some extent, bothered by his inability to throw to the bases—not spooked, per se, but concerned with it. When he signed with the Cubs in December of 2014, he encouraged the team to do what it took to bring Ross along, too. The Cubs obliged, of course, yanking Ross out of a Padres deal at the last moment. Over the last two seasons, Lester threw well over 90 percent of his pitches to Ross. Ross’ arm gave runners pause when they might otherwise have taken more comfortable leads or run more often. Just as importantly, it seemed, Ross helped earn Lester extra strikes and kept him from putting runners on first base in the first place.

There’s no question that Ross was an elite pitch framer. From 2008-2012 he was one of the best framers we’ve ever seen. His Called Strikes Above Average rate (the increase in strike probability we attribute to the catcher, after accounting for the influences of other actors) over that span was 0.029. If you built a leaderboard of every catcher season in our database in which players had at least 2,000 framing chances, you’d get a list over 1,600 names long. Ross’ 2008 through 2012 seasons would all appear within the top 54.

By the time Lester and Ross partnered up, though, that skill was fading. Ross suffered two concussions in 2013. Other nagging injuries, including and especially plantar fasciitis, made 2014 hard on him. He was in his late 30s, his body was breaking down, and his framing numbers declined sharply. Over the last four years, Ross’ aggregate CSAA figure was 0.012. That’s still good (of 63 catchers with at least 2,000 framing chances last year, Ross ranked 18th), but his previous performance was at a level no one matched last season.

The magic of the Lester-Ross partnership lived much less in Ross’ glove than we imagined. In other words, Lester was doing much more of the work than we understood. He finished seventh in pitchers’ CSAA in 2016, among the 144 hurlers who topped 100 innings. In 2015, he was second out of 141. In 2014, he was third out of 149. Compare that to the two rough seasons before he and Ross got together. In 2013: 58th of 145. In 2012: 36th of 142. Lester got much easier to frame, and markedly improved his command, late in 2013, and sustained that forward leap.

Lester is one of the most catchable pitchers in baseball, so although framing is the primary job of catchers a huge majority of the time, arm strength was and ought to have been the primary concern with Lester on the mound over the last few years. There’s no question that Ross and Lester had a helpful synergy and there’s no perfect way to measure their relationship. However, we can say with some confidence that Lester’s command made Ross’s good-not-great framing more than sufficient. Perhaps for the same reasons, Kyle Hendricks (one of the few pitchers with demonstrably better command than Lester, though perhaps not quite the same specific ability to fool umpires that Lester has) didn’t require elite framing work last season, either.

The Cubs did have a pitcher who needed that kind of help, though. Over the last three years, Jake Arrieta has CSAA numbers as bad as Lester’s are good. He’s a pitcher who relies on the deceptiveness of his delivery and the sheer nastiness of his stuff in order to succeed. To avoid walking people and getting into trouble, Arrieta needs a catcher who can corral his stuff and earn him a large number of extra strikes. That’s why his relationship with Miguel Montero has been at least as important as Lester’s relationship with Ross. Since joining the Cubs, Montero has been what Ross was during his heyday. His aggregate CSAA over the last two years is 0.023, and his CSAA of 0.028 last season was the best in baseball.

Montero’s arm is pretty weak, a problem compounded by Arrieta’s difficulty holding on runners. Still, on balance he’s been what Arrieta has needed. As a Cub, Arrieta has worked somewhat extensively with five catchers: Montero, Ross, Willson Contreras, John Baker, and Welington Castillo. His walk rate with Montero is 6.3 percent. His walk rate with the other four is 8.2 percent. Opponents have reached base at a meager .253 clip when Arrieta has been paired with Montero, in over 1,100 plate appearances.

This is an element of our new command and control data (and our tunneling data, too, as we can explore some other time) that will bear watching. Since the first catcher framing numbers landed in the public eye several years ago, we have known that we were looking at just one part of the equation. The pitcher and the catcher work together to shape the strike zone; neither party does it alone. The better a pitcher’s command the more a team can afford to make tradeoffs (like emphasizing throwing arm or even the better offensive player) behind the plate. The better a catcher frames pitches, the more a team can afford to take raw stuff over pinpoint command.

In today’s game, catchers who play their home games outside the state of Missouri aren’t expected to catch 130 times a year. That keeps them fresh both at and behind the plate, and minimizes injury risk, but it also means that backup catchers play more than ever. (A record 53 players appeared behind the plate in at least 50 games last season.) Because of that, teams should stop thinking of catchers as starters and backups, even though one guy will usually catch considerably more often than the other (or others, the Cubs would remind us).

Instead, they should look for ways to pair pitchers and catchers up to optimize synergy and (perhaps more importantly) minimize the number of days on which they field a below-average battery overall. Catchers can and should be thought of as part of the starting pitching rotation. Back when Ross was a framing god, he shared time in Atlanta with Brian McCann, another excellent framer. The two added about 195 runs of value to the Braves’ ledgers from 2009-2012, helping those teams average 90 wins. It wasn’t all Ross and McCann, though.

Those teams were also loaded with pitchers who scored well in CSAA in their own regard: Derek Lowe, Tim Hudson, Jair Jurrjens, Kenshin Kawakami, Kris Medlen, and others. As Bill James recently noted in a piece at his site, the Braves tried a number of different matches between Ross, McCann, and their various pitchers during that time, but tended (or seemed to tend) to make their determinations very much on the pitchers and catchers themselves, and not on gaining the offensive platoon advantage.

Montero was on one division-winning team before joining the Cubs, in 2011, but the Diamondbacks clearly didn’t understand or value framing for most of his tenure there and he usually split catching duties with poor receivers (Wil Nieves, Tuffy Gosewisch, and an ancient version of Henry Blanco). That didn’t erase the value Montero provided, but it didn’t allow the team to fully leverage it either. In 2009, Montero caught a lot, was brilliant, and saved 24.5 runs through framing. Ross had his best framing season the same year, saving 18.2 runs in a much smaller role. Still, the gap between the team framing runs saved for the Braves (56.6) and the Diamondbacks (24.6) is stunning.

The other catchers in rotation with Montero were average framers, at best. Arizona pitchers had a 4.10 ERA when working with Montero and a 4.99 ERA when working with the mixture of players (Luke Carlin, John Hester, and Chris Snyder) who spelled him. Their staff was also loaded with command guys in Dan Haren, Doug Davis, and Jon Garland. Those three pitched fine, but the runs their secondary catching options could not save them were sorely missed. The team went 48-53 when Montero started at catcher and 22-39 when anyone else did.

With Ross gone, the Cubs will likely slide Montero into his former role, only his partner pitcher will probably be Arrieta. Contreras, with his superior offensive skills and cannon arm, should keep Lester afloat just fine. The issue of Kyle Schwarber’s future at catcher remains unsettled, but it seems like Hendricks could work with Schwarber. If there’s one easily replicable thing about the defending champions that other clubs should mimic immediately it’s this one. As our understanding of pitching and catching takes on new depth and dimension, we have a better idea of how teams can field winning combinations of players rather than trying to make ideal decisions on an individual basis and forcing them to fit together. One way or another, the Cubs should field an above-average battery nearly every day next season, barring injuries.