After more than a decade of four-man defenses dominating the Premier League, suddenly nearly half of the teams in the league have shifted to a three–center back, two-wingback formation. So, what gives?

José Mourinho is an unparalleled troll.

On September 17, after his Manchester United demolished Everton 4-0, he praised himself, telling reporters that he took a huge risk by playing an attacking lineup.

“I think the Premier League is becoming defensive ... many teams try to play with five at the back plus two or three in the middle,” he said. “Two teams try to match each other and play the same system and it’s 0-0 or 1-0.”

As with all his trolling, Mourinho’s pronouncement is a cleverly balanced mix of a truth, arguable assumptions, and important omissions—all in the service of making himself look good. This time, the omission was any acknowledgement of Mourinho’s career as a staunchly conservative manager. The rest of the league could decide to play with 10 at the back and it wouldn’t change Mourinho’s approach: building unbreakable, attack-shattering defense is what Mourinho focuses on no matter the opponent.

However, Mourinho’s point about the rise of defensive systems with three center backs and two wingbacks is unquestionably true. According to Michael Caley, this season in the Premier League, teams have started some form of back three/five 30 percent of the time. Last season, that figure was 18 percent, and in 2015-16 teams lined up that way only 4 percent of the time. Go all the way back to 2010-11, the dawn of modern soccer stats, and only 1 percent of all Premier League teams lined up with three center backs.

Yet, despite that rise, and despite Mourinho’s claim, the league isn’t necessarily getting more defensive. Goals have ticked slightly downward, with teams averaging 1.27 per game this season after averaging 1.40 last year and 1.35 the year before that. But the number of shots teams have taken has remained mostly steady with squads averaging between 12.8 and 12.9 shots over the past three seasons. The slight dip in goals seems mostly related to a league-wide conversion slump. Teams are averaging only 4.03 shots per game on target, down from 4.33 last season and 4.3 the year before that, despite basically taking the same kind of shots. By the end of the season, the goal-scoring rate should rise to the levels of the past two years.

While the implications of a shift from four to five defenders might seem obvious—just do the math—the actual effects of the shift don’t match up. Instead, the balance of attack and defend has broadly remained the same; it’s the responsibilities of individual players that’s changed.

From Mourinho on down, we still don’t quite know how to talk about the formational change that’s sweeping the game. Simply shifting to three at the back won’t suddenly solve a team’s problems. Every back three shares certain characteristics, but every manager has to figure out how their personnel can best deal with the new issues presented by a bottom-heavy formation. This Saturday we’ll get to see two managers, Pep Guardiola at Manchester City and Antonio Conte at Chelsea, who have found success within the back-three revolution but have done so in profoundly different ways.

How All Back Threes Are the Same

The immediate cause for the explosion of the back-three formation might be everybody copying Chelsea’s success last year: After a mediocre opening six games to the season, culminating in a 3-0 loss at Arsenal, Antonio Conte switched Chelsea to a back three and promptly won 13 games straight and ran away with the league. That switch itself was a reaction to the broader tactical landscape and its vulnerabilities.

Modern teams rely on fullbacks to provide width in the attacking third of the field. The evolution of wingers into wide forwards, tasked with cutting inside on their stronger foot to score goals rather than staying wide to provide crosses, means that wide attacking areas are almost solely the fullback’s domain. They have the inhuman task of somehow both providing attacking impetus while also not leaving space in behind them. It’s a near-impossible task—a soccer field is between 100 and 130 yards long—and one that really only a couple of players in the world are up to. (And, of course, Pep Guardiola has coached most of them: first Dani Alves at Barcelona and then David Alaba and Philipp Lahm at Bayern Munich.)

When a human fullback inevitably gets caught upfield, he leaves his center backs in impossible positions. Here’s a particularly memorable example from Arsenal’s 3-1 home loss to Monaco in the Champions League way back in 2015:

The entire disastrous cascade starts with right back Héctor Bellerín being all the way in Monaco’s penalty area when Arsenal lose possession. What’s less obvious, but equally as important, is that left back Kieran Gibbs is also in the penalty area. As the play unfolds, center back Per Mertesacker is left flailing in space without Bellerín to protect him, and fails to stop the point of attack. His defensive partner, Laurent Koscielny, then has to rotate over, but without Gibbs behind him, there’s simply nobody left to pick up Dimitar Berbatov and prevent him from scoring. It’s exactly the kind of counterattack that playing with two advanced fullbacks leaves a team exposed to, and it’s exactly what playing with a back three is supposed to help prevent.

With three defenders, even when both wingbacks are attacking the opposition’s penalty area, there is still ample cover. Were Arsenal playing with a back three, after Mertesacker was beaten there would’ve still been two other defenders to rotate and cover. Since this requires center backs to cover untraditional ground for the position, many teams now play at least one player who isn’t a pure center back. These players will often be called upon to step higher up the field toward the ball or to cover for a fullback in wide areas, so players who are defensive-minded fullbacks of defensive midfielders have taken up new roles: César Azpilicueta at Chelsea, Nacho Monreal at Arsenal, Geoff Cameron at Stoke, and Eric Dier at Tottenham were all hybrid players who now regularly slot into three-man defenses.

The other tactical trend that these defenses are responding to is the increased emphasis on teams attacking the half-spaces. A tactical generation ago, starting when Jürgen Klopp first led Borussia Dortmund to surprise Bundesliga titles in 2010-11 and 2011-12, attacking the half-spaces was all the rage. The idea was to attack the space between the wing and the center of the field, because it both made it harder for defenders to close off passing lanes, and also made it easier on attacking players to preserve different options. Done well, it gives a defenses fits. If a fullback pinches in to defend a player in a half-space, he leaves an entire wing open; if a center midfielder slides out, he creates a hole in the middle of the field; and if a central defender steps up, he leaves acres of space in behind. With three center backs, that problem solves itself: the middle center back worries about the middle of the field, while he left center back and right center back are each responsible for their own half-space.

Like a matchup zone in basketball, which assigns specific areas for a player to patrol but then asks them to match up man-to-man within that area, soccer teams are employing a game plan that’s designed to take away the spaces that new attacking schemes want to exploit.

How All Back Threes Are Different

Any formational shift requires trade-offs elsewhere on the field. How teams handle these trade-offs is where the tri–center back formations start to differ. The teams facing off in this weekend’s marquee Premier League match, Manchester City and Chelsea, both will play similar formations, but they’ll do so for wildly different reasons.

The back three leaves teams more protected against the end of a counterattack, with that extra defender to rotate and protect the goal, but more vulnerable to the start of one. When you have one less player to do it with, it’s harder to counter-press and win the ball back immediately after losing it.

At Manchester City, Guardiola basically ignores this problem by relying on an extreme attack to pin defenses back and then just press with one fewer man. If it looks like the press might get broken, they foul. City are averaging 10.7 interceptions per game according to WhoScored.com—that’s not a huge amount, 10th in the league. But, taken in concert with the fact that they have 64.7 percent of the ball, by far the most in the league, that’s a huge number. They pin you deep, make you try to pass it out, and then take it away when you do. So far, the plan has worked as well as possible: City’s league-leading 21 goals might be getting all the headlines, but they’ve been equally stellar defensively. Not only have they conceded only two goals, the fewest in the league, but they’ve also conceded only 41 shots, 10 fewer than any other team. (Reminder: City switch between systems a lot and these numbers encompass all of them, including the times where they play with the more traditional back four.)

Chelsea, on the other hand, sit and defend deeper. They only play with two real center midfielders, but those two, N’Golo Kanté and Tiémoué Bakayoko, are incredibly skilled at not only winning the ball, but at its quieter cousin: preventing ball progression. Effective defense is not always about tackling; in fact, usually it’s not. Despite often being outnumbered by midfield trios, Chelsea’s midfielders excel at taking away passing lanes, and making teams slow their attacks and regroup. Kanté and Bakayoko’s mastery of transitions then allows their teammates to get back behind the ball and set up a final-third defensive wall that’s close to impenetrable.

Only 4 percent of the shots they concede come from inside the 6-yard box, second fewest in the league. In total, 43 percent of the shots they concede come from outside the penalty area. That’s 10th overall, but the top of the list is dominated by teams from the bottom half of the table that consistently pack the box and cede possession. Chelsea’s shots-outside-the-box ratio is better than those of Manchester United, Liverpool, Tottenham, and Arsenal.

The numbers bear out the stark difference between City and Chelsea. At Statsbomb.com, Will Gürpınar-Morgan found that teams simply couldn’t pass the ball against Manchester City, no matter where on the field they were. Chelsea, on the other hand, were happy to concede passes until you reached their final third, and then they simply shut it down. These are last year’s stats, but the same basic principles apply this season:

Statsbomb.com

Ultimately, City use their hyper-aggressive pressing system and successfully gamble that pressured defenders won’t be able to play passes to the midfielders City leaves open. Chelsea rely on a couple of special players to slow the opposition down while the defense builds its fortress around the penalty area.

The two teams are also as different as can be in attack. Playing an extra defender means sacrificing a player somewhere else on the field, and then figuring out how to make up for that sacrifice. Chelsea rely on the passing of their back three to keep the attack humming. Since neither Kanté nor Bakoyoko is a difference-making passer, Chelsea uses David Luiz and Azpilicueta to create with the ball at their feet. Azpilicueta has four assists, all to striker Álvaro Morata. He often steps far enough into midfield to cross the ball to the striker lurking off of defenders’ back shoulders.

Luiz, meanwhile, handles the brunt of the responsibility for the forward passing that the midfield lacks. Chelsea often moves the ball upfield through Luiz finding a pass to a forward and bypassing the midfield entirely.

Guardiola, on the other hand, would likely rather have several body parts surgically removed than build an attack around a center back playing directly to a forward. In contrast to Chelsea, City pack the midfield with Fernandinho, Kevin De Bruyne, and David Silva. When City play with a back three, they don’t ask their center backs to play risky longer passes to open up defenses. Rather, they ask them to carry the ball forward, draw defenders to them, and create the pockets of space for midfielders to slip into and receive the ball. City’s defenders play tons of passes. Of players that have played at least three games, John Stones, Nicolas Otamendi, and Vincent Kompany are first, second, and fifth among defenders in completed passes per 90 minutes—all while completing over 90 percent of their passes.

Both Manchester City and Chelsea have clear plans for how to best utilize their personnel to mitigate the effects of adding a defender—and to make this formation work, that holistic approach is necessary. Tottenham, for example, employ an aggressive press somewhat similar to City’s, but they have an attack that falls somewhere between City and Chelsea: one defender, Jan Vertonghen, strides forward with the ball at his feet, while Toby Alderweireld plays long and direct passes. The result: Spurs can manage a dangerous attack despite sometimes playing as many as six players who might be classified as defenders. Arsenal, on the other hand, have struggled to figure out the right balance, especially on the defensive side of the ball. As has often been the case for Arsène Wenger’s team no matter what formation and personnel they play, their midfield gets overrun against the better teams in the league. Without the supercharged defensive skills of Chelsea’s midfield, the Gunners need a better plan for protecting the center of the field, where they are frequently outnumbered.

As teams increasingly deploy a back three across the Premier League, their success or failure will be dictated not by whether they play that way, but by how they do it. Teams like Chelsea and Manchester City can play differently and both have success: Chelsea won the league last year by seven points, while Manchester City have a plus-19 goal differential through six games. But switching to a back three isn’t a magic bullet. Done right, it can allow players with previously undervalued skill sets to shine, and fill in the positional cracks that many teams find themselves stuck with when playing a back four. Done poorly, it’s like rearranging the deck chairs on the Titanic: The piece are all in new places, but the problems still haven’t gone away.