Five years ago, if you were a slot receiver, you were a niche player. Sure, Wes Welker caught 80 more passes than anyone else in the NFL from 2007 to 2012, but he was the apex of an archetype: the small wideout who couldn’t hack it on the outside.

Fast-forward to today, and the old slot receiver prototype no longer exists. They can be smaller, quicker guys like Julian Edelman, Doug Baldwin, or Randall Cobb. They can be taller guys like Marques Colston or Jordan Matthews. They can be physical guys like Larry Fitzgerald, Eric Decker, or Anquan Boldin. Or they can be anything in between. Whatever they look like, they’re more prevalent than ever.

NFL offenses continually evolve toward efficiency. As teams try to exploit all the space on the field, the three-wide-receiver set has become far and away the league’s most frequently utilized personnel grouping. According to Football Outsiders tracking, teams employed “11 personnel,” which features three receivers, a tight end, and a running back, on 55 percent of all offensive snaps in 2015.

That look appears more than twice as often as the next most popular grouping (two receivers, two tight ends, and one running back), and it’s more than five times as common as the third-most-used personnel group (two receivers, one tight end, two backs). The usage of the three-receiver set has increased every year since 2010.

The reason is simple: 11 personnel increases an offense’s ability to pass while maintaining the option to run. This is where the slot receiver comes in.

As teams eschew running the football in favor of passing more and more, slot receivers have frequently replaced an extra tight end or running back in offensive formations. In turn, many defenses respond to the threat of a dynamic third receiver like Cobb by swapping a linebacker or defensive lineman out in favor of another defensive back. Taking one of those key run defenders off the field then makes it easier for offenses to run.

Slot receivers run better routes than most tight ends and have much better hands than almost any running back. They’re versatile, dynamic chess pieces that offensive coordinators can use to create mismatches. And they’re only going to become more important.

There are three basic types of receiver in an NFL playbook, the “X,” the “Z,” and the “slot.”

The X receiver (the “split end”) is usually a team’s true no. 1; picture Dez Bryant, Julio Jones, or DeAndre Hopkins. In most sets, the X is lined up wide on the opposite side of the tight end, is tied to the line of scrimmage, and will often have to fight off defensive contact without creating any momentum for himself. The Z receiver (the “flanker”) lines up on the opposite side of the formation from the X, typically off the line of scrimmage, frequently going into motion before the snap. He’ll see a lot of man coverage against corners, but because he can go in motion, he’ll sometimes get passed off to linebackers and safeties on in-breaking routes.

The slot receiver, whose designation varies based on the playbook (A, U, W, or S), will see coverage from all kinds of defensive players, including linebackers, safeties, and nickel cornerbacks. They’re placed all over the formation: You find them close to the offensive line (a “nasty split”), out wider toward the sideline in a trips formation, or sometimes in the slot-back spot just off the butt of the tight end or tackle. They typically line up off the line of scrimmage and can move around before the snap. They run a variety of routes, but their average depth of target is generally lower than their counterparts on the outside.

Despite the varying molds for today’s slot receivers, there is one common theme: They all run their routes from the middle of the field, and their point of view presents a completely different kind of challenge compared to what outside receivers have to overcome.

All receivers run “option” routes, in which the onus is on the pass catcher to read the defense and decide which route to run and how deep to run it. But for slot guys, the decisions to convert or completely change these routes tend to be much more complex. The middle of the field is crowded with moving parts — dropping linemen, blitzing or covering linebackers, the lurking secondary — so in a matter of seconds, they have to diagnose the defense, get on the same page as their quarterback, then be willing to improvise in order to find space. When done right, the dynamic and reactive nature of the routes they run is a major headache for defenses because their movement becomes so unpredictable.

Since slot receivers can move around a formation before the snap, it’s also harder for defenses to disguise their coverages: If a defender follows the receiver across the formation, that usually suggests a man-to-man scheme (and the opposite can suggest a zone). Plus, the increasingly varied sizes, speeds, and styles of these receivers make them hard to match up with in any standard way. One week you’re scheming to defend the 6-foot-3 Fitzgerald from the slot, the next week it’s a tiny speedster like Cobb or Jarvis Landry; it’s rare to have one defender versatile enough to handle every type of slot receiver.

In some cases, defenses will put a cornerback or designated nickel defensive back out on the field to cover the slot. When offensive coordinators or quarterbacks see this, they’ll often run quick “man-beater” routes, a favorite for Tom Brady and the Patriots with Julian Edelman. Aqib Talib is a big, physical corner, but is no match for Edelman’s insane quickness. When the Patriots get Edelman matched up in man coverage against anyone, the pass is almost always going to go his way. He fakes the drag route before sticking his foot in the ground and running the out.

(All videos are from the 2015 NFL season.)

Randall Cobb has a similar role in the Packers’ offense, and here he runs the same route from the other side of the field against veteran corner Captain Munnerlyn. Before the snap, you can see Aaron Rodgers subtly motion to Cobb that the ball is coming his way. They have the man-to-man matchup they’re looking for, and even with relatively tight coverage from Munnerlyn, Cobb is too quick to be defended in space.

The Seahawks do the same with Doug Baldwin when they get the defensive look they want. Here, corner Jerraud Powers is just helpless against Baldwin’s quick footwork — he has to defend the middle of the field, too — and the sudden stab inside by Baldwin is enough to fool Powers and gain the separation he needs.

When man-to-man coverage isn’t an option, defenses look to match up with slot receivers in a variety of zone looks. That means that linebackers are often tasked with defending slots over the intermediate middle, and most of the time they’re simply hoping to discourage a pass by getting into passing lanes. It doesn’t always work, particularly against quarterbacks who can thread the needle. Brady is one of those quarterbacks.

Danny Trevathan is one of the best coverage linebackers in the league, but against this route, he would’ve had to anticipate the depth and width of Edelman’s route perfectly to make the play.

The Seahawks ran this same concept against the Panthers, who rely heavily on coverage from their linebackers, Thomas Davis and Luke Kuechly. Here, Davis drops into a zone coverage over the seam but can’t cover enough ground when Baldwin runs past him.

When neither man nor zone is working alone, teams try a little of both: Earlier this year, the Packers used a combination of man and zone coverage against the Cardinals in their divisional-round playoff matchup — but Fitzgerald still got the best of them. Casey Hayward lines up to try to force Fitzgerald inside — his pre-snap alignment is to Fitzgerald’s outside foot, and he opens up looking to take away anything outside. Fitz runs a crossing route, and Green Bay is looking for it. Clay Matthews is hoping to jump into the passing lane, but he misplays the timing, and Fitz sneaks past him, catches the pass, and runs for a big gain.

Sometimes safeties get to partake in the fun, as teams will ask them to manage the difficult task of both walking down into tighter zone coverage and not getting beat deep. Against the Cardinals, Baldwin sets up his route like he’s going to go deep on a corner route before cutting it off with a deep drag. Safety Rashad Johnson has to respect that deep route, and he slips while trying to recover.

Even when safeties are quick to break on slot receiver routes over the middle, it’s often too late. Here, the Patriots run a route combination with Edelman and Rob Gronkowski that draws the linebacker (no. 90 Josh Mauga) out of the passing lane in the middle of the field. Chiefs safety Ron Parker has to break on the route and make the tackle, but when this offensive play is run correctly against this defense, it means New England is still getting a big gain.

As slot receivers have replaced tight ends and running backs in offensive formations, we’ve seen teams creatively use the skill sets that these players possess. Colston made a career out of catching passes up the seam from Drew Brees in New Orleans, as the Saints used Colston’s excellent height and length against shorter defenders to their advantage. Boldin may not be fast at this point in his career — hell, he was never fast — but he’s extremely physical and understands how to box people out to make contested catches.

Slot receivers are still integral on third downs, and they are even becoming more dangerous as touchdown makers. Baldwin tied for the NFL’s lead in touchdowns — an honor typically reserved for outside X receivers or stud tight ends — while operating primarily from the slot. Decker, who often ran out of the slot, was a touchdown machine (12 scores) for the Jets from the slot in 2015, and Cobb scored 12 times in 2014 while running primarily from the slot for the Packers.

Slot receivers are more dangerous than ever, and teams are finding ways for their elite outside playmakers to run routes from the inside to exploit the mismatches it presents. And as the three-wide-receiver personnel set becomes more entrenched, teams will continue to adapt to the ways they use their slot receivers. The approach might change, but one thing will continue to be true: The slot is no longer a second-class citizen among the receiver elite; it’s a weapon that every team will employ.