49ers: A Defense in the Mold of a Rival, Part 1 — the 4-3 Defense

This is part one in a series examining the new 49ers 4-3 defense under defensive coordinator Robert Saleh.


The 49ers recently hired former Jaguars linebacker coach Robert Saleh to be the new defensive coordinator under head coach Kyle Shanahan.

The hire signals a shift from the 3-4 to 4-3 base defense. Prior to the becoming the Jaguars linebackers coach under former head coach Gus Bradley, Saleh worked as the quality control coach under Bradley for the Seattle Seahawks.

It makes sense then, the new Niners defense will be built in the mold of the Seahawks defense that was crucial to their 2013 Super Bowl victory.

The 49ers will reportedly be running a base 4-3 “under” defense (1-gap), as well the 4-3 “over,” both with 3-4 (2-gap) elements in it.

We’ll also cover the 3-4 “bear” front. It’s unlikely the 49ers will remain in a traditional 4-3 or 3-4 front for most of the games they play, due to the nature of opposing offenses using more shotgun and three wide receiver sets than normal, forcing defenses into nickel and dime sub packages.

But the switch to 4-3 is still a significant departure for a team that was largely built as a 3-4 defense for the last decade.

All of their recent drafts — including selecting Arik Armstead and DeForest Buckner the last two years in Round 1 — have tended toward drafting defensive players that fit the 3-4 mold.

However, this defense isn’t new to San Francisco and in many ways. Part of this defense’s legacy began under Bill McPherson when he was the defensive line coach under coached under George Seifert in the 1980s and later becoming the assistant coach in the 1990s under Seifert.

In 1995, current Seahawks head coach Pete Carroll became the 49ers defensive coordinator and incorporated elements he learned from coaching under Monte Kiffin before embarking on his current path.

Before we get into the alignment of the “under,” “over” and “bear” front defenses, it will be necessary to distinguish between 1- versus 2-gap responsibility.



It can be hard for the average fan to distinguish between a 1-gap and 2-gap defensive lineman.

You shouldn’t take my word for it when I say this or that guy plays this or that assignment. One way to distinguish is to get your hands on some of the many playbooks that are floating around the internet.

In this vintage 1992 49ers defensive playbook, you can see some of the origins of this defense. One term they commonly used is “base 2-way read, control strong and weak A-gaps.”

Another way to distinguish on film what specific gap techniques the defensive line plays are by watching the technique a particular defensive lineman uses. The key indicator in the rush technique is the hand placement of the defender. This video goes into more detail on how this technique is coached:

For 1-gap rush technique, the defender’s hand placement is on the sternum and outside arm of the offensive lineman in the gap with the defensive lineman’s head in that same gap.

For 2-gap, the defensive lineman looks to rush head-up on the blocker while his hands control the blocker’s armpits, thumbs-up, elbows turned in. The defensive lineman looks for ball, sheds the blocker and fills that gap to either side of his blocker.

The 4-3 under hybrid defense that Saleh is expected to install is a direct descendant of Carroll that has evolved over time with each stop in Carroll’s coaching tenure. The current iteration of it was perfected by the Seahawks from 2011 – 2013 before Bradley left for Jacksonville.

Writing for the Seattle Times, Jerry Brewer shed some insight into the defensive mind of Pete Carroll when wrote:

It’s a defense that can be explained, at its most elementary level, as a hybrid scheme that mixes 3-4 concepts in a 4-3 front. But what makes the Seahawks stand out is the coaches’ willingness to play off the diverse skills of their players.

That flexibility has allowed the Seahawks to identify underrated talent that doesn’t fit into a mold, to focus on strengths instead of weaknesses in player evaluations and to create an oversized, yet not lumbering, defensive unit.

Carroll further says that “We mixed the concepts of one-gap football and two-gap football in a very unique way in San Francisco,” The defense can be expected to be a “one-gap-and-hold” that utilizes certain defensive linemen to “hold” rather than penetrate their gap (2-gap principle). This allows the defensive line to clog any potential running lanes.

So what is the “4-3 under” defense and what does it seek to do?”



4-3 Under defenses try to do two things: stop the run and pressure the pass. It traditionally uses one-gap techniques to do so, but as we’ll see in a bit, this hybrid defense can and will use combo one-and-two gap front, which means that it relies on penetration and disruption of the offensive line.

The 4-3 under asks each defensive lineman and linebacker to be responsible for a single gap, which increases player’s ability to play instinctively without overthinking, which was a common problem with former defensive coordinator Jim O’Neil’s defenses in Cleveland and last season with the 49ers (for more background, see here, here and here).

It will be more common to see some two-gapping by a defensive end (4 or 5 tech defensive end in an under front, 6 tech in an “over” front) and the nose tackle from the 1-tech.


In an under front, depending on where the strength of the offensive formation is, one defensive end will play the 4 or 5-technique and the nose tackle will play the 1-technique and line up to the closed side (or strong side with the tight end) of the formation. Also on the strong side, the “SAM” or “OTTO” linebacker, will line up over the tight end.

The nose tackle and defensive end will usually be the best run-stoppers while the SAM linebacker must have the ability to set the edge against the run and drop in coverage consistently.

On the other side of the formation (the weak or open side), will be the defense’s two best pass-rushers, the 3-technique defensive tackle and what is known as the “LEO.” The LEO lines up to the outside shoulder of the weak side offensive tackle, leaving the “MIKE” and “WILL” linebackers to handle the strong side ‘B’ gap and the weak side ‘A’ gap, respectively.

In the gif below, pay attention to the technique of the 2-gap defensive linemen:

4-3 OVER

Another variation we are likely to see, but not as much of, is the 4-3 “over” front.

In the “over” front, the 1-tech nose tackle is away from the strong side, lining up in the weak side “A” gap with the LEO also to that side. On the strong side, there is a 3 technique tackle and either a 5 or 6 technique defensive end.

Another giveaway is the depth of the linebackers, with the SAM playing in the 2nd level with the MIKE and WILL linebackers. This is much more of a traditional 4-3 front.


The 3-4 “Bear” front (or 46 defense) was popularized by the 1985 Bears and is currently the base defense of the Virginia Tech Hokies under defensive coordinator Bud Foster, though they use a 4-3 variation of it. Same basic principles apply.

The bear front’s main advantage is in stopping the inside running game. The 5-man front across the line of scrimmage make it hard for an offense to create double teams at the point of attack on gap scheme runs and prevent those same linemen from getting vertical into 2nd level blocks in the zone running game.

It is run with less frequency by the Seahawks under Gus Bradley, but it nonetheless should be something 49ers fans can expect to see here an there.

(Diagram adapted from Mike Chan Field Gulls)

The bear front alignment has a nose tackle in the 0-technique, 2-gap position. On either side of him are 1-gaping 3-technique defensive ends.

To either side of the defensive ends are the SAM and LEO edge defenders who can either rush or drop into pass coverage.

In part two of this series, we’ll look at the base pass defenses that will be employed, the cover 3 and cover 1.

All images courtesy of NFL.com and NFL GamePass.

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