Football is littered with specialized terminology. From deep comeback route to 0 technique nose tackle, commentators rarely get to explain everything you need to know before the next play. Inside The Pylon’s glossary was developed to give fans a deeper understanding of the game through clear explanations, as well as image and video examples. Please contact us with any terms or phrases you’d like to know more about.
4-3 Under Front
A 4-3 Under Front consists of four defensive linemen and three linebackers that shifts the strength of the defensive line ‒ and, in particular, the 3 technique defensive tackle ‒ to the weak/open side of the offensive formation. Generally a one-gap, penetrating front, the 4-3 under aligns five defenders (four defensive linemen and the strong-side, or Sam, linebacker) near the line of scrimmage, which can give the appearance of a 3-4 defense. This front is commonly deployed against offensive personnel that utilizes tight ends and/or a fullback.
Identifying The 4-3 Under
The key to identifying a 4-3 under front involves recognizing the alignment of the interior defensive lineman as well as the positioning of the Sam linebacker.
Along the interior of the under front, the nose tackle will shade toward the A gap on the strong/closed (or tight end) side of the offensive formation. Conversely, the 3 technique defensive tackle will align on the weak/open side of the offensive formation. He is often referred to as the under tackle within this defensive front.
Meanwhile, the Sam linebacker will typically align close to the line of scrimmage on the strong/closed side ‒ often directly over or shaded slightly outside the tight end ‒ positioned similarly to a defensive end, although maintaining a two-point stance.
As pictured above, the alignment of the remaining defenders along the line of scrimmage in the under front usually has the strong/closed side defensive end lined up just outside the offensive tackle in 5 technique, and the weak/open side defensive end lined up over air well wide of the offensive tackle (often referred to as the LEO, which abbreviates “lineman end open”).
At the second level of the defense, the middle (Mike) linebacker and weakside (Will) linebacker fill any bubbles (remaining uncovered gaps), aligning four or five yards off the line of scrimmage but directly over their gap assignment, unless tandeming on an exchange with a defensive lineman.
Strength of Front
Although more defenders are aligned to the strong/closed side, the two weak-side defensive linemen in the 4-3 under are often the most dynamic playmakers because of their relative isolation, and unlikelihood of being double-teamed. In turn, the Will linebacker ‒ usually the quicker of the three linebackers on the field ‒ is well protected by the defensive linemen positioned in front of him and is often able to flow freely to the strong-side to provide support in the running game.
Seahawks 4-3 Under Front
The presence of the fullback in the backfield theoretically creates an additional gap for the defense to contend against somewhere along the closed side of the formation. By walking safety Kam Chancellor (#31) into the box (over the open side B gap bubble), weak-side linebacker K. J. Wright (#50) can read and flow toward the direction of the ball carrier and account for the extra gap.
On the play, the Rams two-back power run targets the C gap by utilizing a double-team on the 5 technique defensive end Michael Bennett (#72) to open up the point of attack in concert with a pull block from the backside offensive guard:
The left tackle and tight end work the combo block on Bennett, but neither can slide off him to the second level as the defender out-muscles the blockers, driving them laterally down the line of scrimmage. The pulling right guard coming from the backside leads up into the hole looking to trap block one of the Seattle linebackers, but because of both Bennett’s effort and Wright’s free rein within the under front alignment, the defense has the numbers advantage and stuffs the run.