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Now that Robert Saleh's hiring has (finally!) become official, it seems appropriate to look at his 4-3 under defensive scheme and the personnel the 49ers have on hand to fill the specific roles. A couple of 49ers writers have already offered their take on how well the 49ers' personnel fit their new scheme, but I'd like to spend a little more time discussing the scheme and the responsibilities of each position within the scheme. I also have some differing views on the players and their positions.

THE BASICS


To talk defenses, we need to talk gaps. A "gap" is the space between (or directly to the outside of) offensive linemen. "A" gaps exist on either side of the center. "B" gaps are between the guards and tackles. "C" gaps are directly outside of the tackles (or between a tackle and a tight end), and "D" gaps are the aiming points of wide sweeps that look to outrun the defense to the numbers or sideline. There are four gaps on each side of the offense, and a defense must account for all eight gaps. Traditionally, 3-4 defenses had been able to save money because they generally employed oversized defensive linemen (DL) who were great at holding their ground, but poor at gaining ground. These players offered little in the way of a pass rush, but they proved their worth with their ability to two-gap. When a defensive lineman two gaps, he controls the offensive lineman (OL) across from him, essentially taking responsibility for the gap on either side of that man (a DL controlling a guard is responsible for the A gap and B gap to that side of the offense). A two-gapping defensive lineman should be massive and strong, and having exceptionally long arms is a huge leverage bonus. By two-gapping with two or three of their defensive linemen, 3-4 defensive coordinators ideally free the linebackers (LBs) to run to the ball and tackle downhill. IF all 3 DL two-gap, six of eight possible gaps are accounted for by three players.

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"One-gapping" most commonly describes when a defensive lineman aligns to a shade (inside or outside shoulder) and attempts to shoot directly through the gap they are shaded to at the snap of the ball. Some stunts may require the DL to cross the face of an OL to threaten a different gap, but the general idea is to get defensive linemen through the offensive line into the offensive backfield to attack and disrupt. When the defensive line one-gaps, the LBs are no longer simply free to flow to the ball, as they are also assigned a specific gap to fill. The 4 DL and 3 LBs combine to account for seven of the eight gaps, which requires a defensive back to rotate down to the box to fill the eighth gap. One-gapping across the defensive line allows a defense to play aggressively, but it can also limit coverage options in the secondary, since one safety is generally down in the box and coverage schemes are often limited to one-high safety looks (cover 3 and cover 1). In a one-gapping scheme, the DL are typically the playmakers, and the LBs fill behind the DL to "make them right," which essentially entails filling the remaining gaps after the DL penetrate through the line and disrupt the offense.

Traditionally, 3-4 defenses two-gap with their DL and 4-3 defenses one-gap. The 3-4 OLBs provide the quick-twitch get off and bend around the edge that is typically associated with defensive ends in a 4-3 defense. Having two such LBs on the edge allows the defense to make the offense guess which of the two LBs will be the fourth pass rusher on any given play (assuming that the three DL are typically rushing). The three-technique defensive tackle (3T or 3T DE) in a 4-3 is generally a quick-twitch athlete with a big body and fluid hips who can beat interior offensive linemen instantly off the snap of the ball and wreck the QB's mesh with his RB or quickly disrupt the QB in the pocket (think Warren Sapp and Aaron Donald). 3-4 DEs are typically the large, strong, and long leverage monsters detailed in the description of two-gapping.

SCHEME EVOLUTION


Something pretty cool has happened as coordinators of both of these defenses seek to cover their weaknesses: the schemes have demonstrated the ability to merge concepts to combat offenses that have mastered attacking each scheme in its most basic form. 3-4 defensive coordinators have realized they can disrupt run schemes and ratchet up their aggression up front by allowing one DL at a time to one-gap. By two-gapping with two DL and one-gapping with one, the defensive line still accounts for 5 gaps with three men, but one of those men is penetrating vertically, picking off pulling linemen in a gap/power scheme or disrupting lateral flow in a zone scheme (imagine Justin Smith or Arik Armstead blasting through the line virtually untouched to tackle a running back immediately after a handoff). By varying which DL is one-gapping, the defense can make the OL play with greater caution, knowing that each DL could explode forward off the snap of the ball or snap their hips, lock out their arms and neutralize the offensive push to hold ground. Correspondingly, 4-3 defenses have started to two-gap with one DL to account for all eight gaps with the seven defenders who comprise the defensive front. This allows the four defensive backs greater flexibility in coverage, allowing two-high safety looks, like cover 2, cover 4, and cover 6 (cover 2 on one side of the defensive backfield, cover 4 on the other).

THE 4-3 UNDER


The 4-3 under scheme will look familiar to 49er fans, as three of the defensive line positions look at first glance to be interchangeable with the three defensive line positions within a 3-4 defense. The "under" designation indicates that the defensive line has shifted away from the offensive strength (called an under shift). A typical 4-3 defense has two defensive ends on the outside shade of each offensive tackle (aka five-techniques or 5Ts) and two defensive tackles, one on the outside shade of the strong side guard (three-technique) and one on the weak side shade of the center (one technique).

In a 4-3 under, each DL moves one gap away from offensive strength (often defined as the alignment of a tight end). The strong side DE will usually shift from outside the offensive tackle (5T) to either a head up alignment (four-technique or 4T) or an inside shade of the same offensive tackle (4i, where "i" means inside). The 3T moves from outside the strong side guard to a strong shade of the center, essentially becoming the new one-technique. The weak side DT shifts from the weak shade of the center to the outside shoulder of the weak side guard, assuming the responsibilities of a typical 3T. Rather than have two players out of position (the 3T playing as a 1T and vice versa, the two DTs simply switch sides, so the 3T remains as such, just on the weak side, and the 1T essentially only moves from the center's weak side to his strong side. The weak side DE shifts from a close outside shade of the offensive tackle (5T) to a wide shade in space, still in a three- or four-point stance. This shade is often referred to as a "seven" or a "wide nine," indicating that the end is far enough outside of the tackle to be playing in space. This position is generally referred to as the LEO (think of Charles Haley's "Elephant" position with the classic 49ers defenses). The strong side OLB (or SAM) aligns on the line of scrimmage (LOS) head-up or on an outside shade of the tight end, and the remaining LBs (MIKE and WILL) line up 3-4 yards deep, across from the guards.

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Pete Carroll has described his defense as being a 4-3 defense that uses 3-4 personnel. For years, the strong side DE was 320+ pound Red Bryant, who played frequently in a 4T. This alignment, combined with his size, allowed Bryant to two-gap, freeing the other DL to one-gap. It is a HUGE bonus if the 4T is capable of providing anything in the way of a pass rush. Typically, that 4T and/or the 1T will two-gap, eating double teams and allowing the defense to cover all of the gaps without committing a safety immediately to the run, which is precisely what the 49ers did with Vic Fangio as their defensive coordinator. Incidentally, Fangio often deployed the 49ers in an under front, but he kept Aldon Smith (the de facto LEO) in a two-point stance to preserve the 3-4 look and disguise the fourth pass rusher. If you think of the LEO as a 3-4 OLB with his hand on the ground, the positioning of the other three DL closely resembles the positioning of the three DL in a typical 3-4 defense.

The one-technique (1T) defensive tackle is named such because he plays on a shade of the center. This player should exhibit the same traits as a 3-4 nose tackle. This player will still most commonly engage the center on the snap of the ball, will still see double teams between the center and a guard on many (most) run plays, and will likely leave the field in sub packages. This player is important, and the complete lack of expectation that this position provides any pass rush should allow a team to fill this spot without breaking the bank.

The 3T can also be a two-gapper, but the 4-3 under is at its most dangerous when this player is an attacking one-gap penetrator. Like a 3T in a more traditional 4-3, this player could be the most freakish athlete on the defensive line, combining size and strength with a quick get off that frustrates interior lineman and wrecks blocking schemes. This player should stay on the field as an interior rusher in sub packages.

If the 3T isn't the most impressive athlete on the DL, the LEO should be. This is the "WOW! Did you see that?!?" edge rusher who explodes off the line, runs the arc, and bends under/around the offensive tackle so effortlessly that it looks like the QB never has a chance to throw the ball. The LEO is aligned wide to place additional strain on the pocket and to provide additional leverage for setting the edge against the run.

I think of the SAM as a failed 3-4 OLB. When a good, long athlete doesn't thrive as a 3-4 OLB, it's typically because they haven't managed to produce pressure off the edge on pass plays. That's not as big of a deal for a SAM. As a strong side OLB lined up over a TE, the SAM must set the edge against the run against what is typically an inferior blocker, compared to an offensive lineman. Should the offense pass the ball, the SAM must be athletic enough to drop into zone coverage (often the flat) or man cover the TE down the field. If the SAM is sent on a blitz, he's most likely rushing against a TE or a back, so he doesn't need to be proficient at beating an OL with his rush.

The MIKE is the man in the middle, stuffing runs between the tackles, pursuing sweeps outside the numbers, and crushing crossing routes in the passing game. MIKE must read and react quickly and get to the football.

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WILL is a weak side LB, but the under shifted front gives the WILL a little more run-stopping responsibility. Will must still be adept at covering backs, tight ends, and slot receivers, but he must also be able to read a guard's path and fill a gap.

The cornerbacks can vary by preference, as rangy deep-third zone players in a cover-3 scheme, or strong press corners with the hips and quickness to stick with wide receivers in a cover-1 scheme. The Seahawks employ long CBs who press at the LOS (as they would in cover-1), but who often convert their press coverage to a deep-third responsibility (typical to cover-3) as the play develops. Kyle Shanahan has communicated that he prefers large physical CBs in the Seattle mold. Physical press technique on the outside makes it difficult for QBs to throw timing routes, forcing them to let the concepts develop. That delay could give the pass rushers time to reach the QB. Conversely, if the pass rushers are able to pressure the QB quickly, they cause him to either take the sack or force a throw into tight coverage, allowing the CBs an opportunity to produce turnovers.

Under Vic Fangio, the 49ers safeties were expected to be interchangeable. No more. While the tendency of the 4-3 under defense to ask one or more DL to two-gap allows the defensive backfield to present a two-high safety shell, the safeties clearly exhibit different physical traits. The free safety has to cover ground. LOTS of ground. Foot speed and lightning-quick read and react instincts allow this man to protect the aggressive CB play on the edges, while providing deep security against post and seam routes.

The strong safety is a thinned out LB who drinks nitrous oxide by the quart. While this player must also cover ground, he is doing so to deliver a message of danger and physicality. This is a rocked-up version of the typical box safety. The fluid hips and nimble footwork typical of a slot defender are sacrificed here, and in the balance the defense gains the swagger of employing a legitimate punisher who has ten yards or more to accelerate into the sort of punishing impact that makes offenses play slower and causes receivers to start thinking long-term. A SS in this mold flies up field to punish running backs who manage to break through the LOS. This player wrecks intermediate crossing routes and eliminates yards after the catch on curl routes and passes to the flat.

As a change-up, the safeties can occasionally switch responsibilities, which drops the SS into deep-third coverage while assigning the FS to patrol intermediate zones. The defense would switch responsibilities to increase their physical presence deep and between numbers, typically when tight ends or large slot receivers are catching passes deep down the seams. A few brutal and well-timed hits as the ball arrives can greatly decrease the enthusiasm with which a receiver extends his body to secure those deep passes. By increasing the strong safety's depth, he also gains a deeper angle of approach on sideline routes, allowing for punishing hits on a route as wide as a sideline wheel. This is exactly what happened when Kam Chancellor came from deep safety to destroy Vernon Davis. Earl Thomas (Seattle's FS) lined up in the box, and Chancellor saw Davis's wheel route develop and met him with frightening momentum and ferocity at the catch point. Chancellor's size and his build up speed combined to wreck Davis, who was never the same as a 49er, especially against Seattle. At first glance, I thought I could see Vernon's soul leave his body, but upon replay I saw that it was simply the rain on his helmet getting scattered into mist by the impact. Playing the SS deep also allows the FS to use his speed, quickness, and instincts to prowl slants and intermediate crossing routes for interceptions.

WHICH 49ERS FIT THE DEFENSE?


With a full understanding of the incoming defensive scheme, it's time to take a look at how the existing 49ers personnel would fill out the depth chart. While depth might be an issue (that's expected), the 49ers appear to need starters at the same two positions they would have been lacking if they had stayed in a 3-4.

Strong Side Defensive End


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DeForest Buckner would be a monster in this role. The Seahawks had Red Bryant, who had the strength to two-gap as 4T. The Seahawks now play Michael Bennett there, and he is a disruptive penetrator in the C gap as a 5T. Buckner has the size, strength, and athleticism to do both. As he continues to work on more consistent pad level, he will get even stronger in his anchor against double teams, while maintaining the potential to burst through on either side of the tackle when he one-gaps. In limited repetitions as an edge rusher this year, Buckner also flashed the agility to flip his hips and turn the edge. This would allow him to provide an effective contain rush if the offense passes when the 49ers are in their base personnel. Quinton Dial makes sense as depth here, but he'd be a far less dynamic presence. His ability to two-gap would provide gap integrity (all gaps accounted for) without safety help.

1T


The 49ers don't have one of these. Like, at all. With Glenn Dorsey and an injured Ian Williams off the books for 2017, the cupboards are bare. Whether the 49ers stayed in a 3-4 or transitioned to a 4-3, they were going to need a nose tackle. The 1T provides everything that a 3-4 nose should, including the ability to two gap and endure double teams. The 49ers need to sign a 1T in free agency and secure another one or two during the draft and undrafted free agency. Brandon Williams and Dontari Poe are two free agents who would instantly lock down the position. Both have the strength to two-gap and the athleticism to one-gap. The 49ers need to find their starter in free agency, because rookie defensive linemen tend to struggle against NFL combo blocks.

3T


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This is where Arik Armstead belongs. In a 4-3 scheme, this player is rarely asked to two-gap, which is one facet of the game that Armstead has struggled with (in fairness, his shoulder injury last year would have greatly limited his ability to hold ground). Even with a shoulder injury, Armstead looked unblockable on the occasions when he was allowed to get off and penetrate immediately off the snap. As an attacking 3T, Armstead could quickly silence doubters and develop into a legitimate star. His strength and leverage would allow him to occasionally two-gap, freeing Buckner to one-gap and keep offenses guessing and off-balance. Ronald Blair has uncommon quickness for a DL, which would serve him well in this role, if he isn't required to slim down to play an edge position.

LEO


The 49ers really, REALLY need a dynamic, quick-twitch edge rusher. That's a polite way of saying that there is no LEO on the roster. The last 49er to beat a tackle around the edge off the snap was Ahmad Brooks, over three years ago, when he instantly crushed Drew Brees to produce a sack-fumble that only the NFL could take away. If he still had that get off and bend, he'd be the 49ers' best bet at LEO, and he still might be, even with the added weight and diminished athletic explosion. The 49ers need help here, in free agency or (and?) in the draft. Melvin Ingram makes a lot of sense in free agency, and there are several promising edge rushers in the draft, including physical freaks Myles Garrett and Takkarist McKinley. The 49ers might believe this position is a last chance option for Tank Carradine, but I would likely use him as a designated edge rusher in sub packages, entering the game when the SAM and 1T leave. The 49ers played with using Blair as an edge rusher last year, so he could be depth here if they slim him down, but I'm skeptical that his lateral quickness would compensate for his lack of elite up field get off.

SAM


Remember that whole thing about a SAM being a failed 3-4 OLB who doesn't give you much as a pass rusher, but who is able to do all of the other stuff? If that didn't scream, "Eli Harold!" to you, you might have forgotten that he exists. Harold bulked up in the 2016 off season, and he now has both the size to neutralize TEs in the run game and the athleticism to cover passes. He will likely never be anything special rushing the passer, but he wouldn't do much of that as a SAM, and he'd likely rush against a skill player as a blitzer, whenever he would get sent after the QB. I don't think the 49ers have much here in the way of depth, but Brooks could play here if the 49ers sign/draft a starter at LEO and Brooks is retained on the roster. I don't think he's the runner he used to be, though, and that would make him a liability in coverage.

MIKE


Um, duh. Navorro Bowman, by every account, is well on his way to recovery, and he will return to lead the 49ers defense. Bowman will continue to meet runners between the tackles and snuff out plays attacking the edges of the defense. Depth here is spotty. While I don't believe Shane Skov has the instincts or athleticism to excel here, he doesn't fit anywhere else. Zach Brown, a free agent out of Buffalo, would be good insurance here if the 49ers are worried about Bowman's recovery, and he could compete with Armstrong at WILL if Bowman is healthy.

WILL


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This position is a near perfect fit for Ray-Ray Armstrong. His length and agility would allow him to restrict throwing lanes in zone coverage, and his background as a safety would allow him to stick to backs, tight ends, and slot receivers in man coverage. He improved remarkably as a run defender in 2016, prior to succumbing to a pectoral injury. There's no one else on the roster who should play here. The good news is that depth players at WILL are cheap. The 49ers could spend a little extra and sign Brown to compete with Armstrong for the starting role at this spot.

Corner Back


Tramaine Brock isn't a popular guy with the 49er Faithful, but he's the best CB the team has in press cover technique. At least, for now. Rashard Robinson is a budding star with gifts and confidence busting out. He's also three inches taller than Brock, whose 5'10" stature might not reach the dimensions the new 49ers leadership is seeking. If the 49ers are serious about getting big at the position, they have Robinson and Dontae Johnson as 6'1" and 6'2" options, respectively. Johnson has flashed big plays, but has struggled to hang onto a significant role in the defense. Will Redmond is a dynamic athlete with fluid coverage abilities and impressive ball skills who is also listed at six feet tall. I haven't seen him in press technique, so I don't know how he'll factor into this competition. Redmond is also an ideal nickel cornerback, as he possesses the agility and acceleration to stick with shifty slot receivers on a two-way go.

Free Safety


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Jimmie Ward. Say it again! Jimmie WARD! This dude, man. THIS! DUDE! When you watch his college footage, you'll wonder what the hell the 49ers were thinking when they made him a CB. At 5'11", he might not even fit the height/length profile the 49ers reportedly now favor at CB. Yes, he can play corner, but he was made to make amazing plays at breakneck speed as a high safety. His strength, agility, and acceleration allow him to stick to WRs in man coverage as a CB, but his instincts shift into light speed when he reacts to plays in front of him. He's the only guy on the 49ers roster with any hint of the play making ability and deep security that Earl Thomas provides to the Seahawks. He high points the ball, out positions WRs on contested catches, and reaches plays no QB would expect him to get to. On top of that, he is both a violent and dependable tackler. His foot injuries don't concern me at all, but his most recent injury was to his shoulder, which is concerning for a safety. Luckily, it was broken clavicle, so there is little cause to worry about the integrity of the shoulder joint, but safeties have to be able to strike forcefully with the shoulder without any concern. The 49ers must decide if Ward's broken collar bone was a freak accident or an indication that concerns about his size (193 pounds) are legit. There are no backups or competition here. Reid has looked more hesitant and indecisive since suffering three truly scary concussions over his first two seasons. As such, his athleticism isn't enough to allow him to cover the ground that a FS should in this scheme. The 49ers should look to the draft for depth, or for a starter if they can't count on Ward here. Malik Hooker from OSU is perfect for the role, but it's unlikely to 49ers would jump on a safety early enough to secure Hooker, with so many other weaknesses (someone has to throw the football) on the team.

Strong Safety


Jaquiski Tartt makes the most sense here. He is the same type of size/speed super freak as Kam Chancellor and he hits a ton (watch his college highlight reel). He runs even faster than Chancellor, but his underdeveloped read and react instincts have him guessing where to go on too many occasions. The comparably simple coverage schemes the 49ers are adopting should allow him more opportunities to think less and play faster, which is a scary proposition for opposing offenses. 221 pounds hitting you at 4.5 40 speed is no joke. If Tartt can speed up his processing, the 49ers would need Reid or Antoine Bethea to hold down the spot until Tartt can take over or until a replacement arrives. Neither Reid nor Bethea can be counted on as a physical presence. Reid hit like a hammer his rookie year, but has adjusted his game to protect against further concussions, while Bethea has never been a big hitter.