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Summer School: Understanding the 4-3 defense

Today, we begin a series we are calling "Summer School." We hope to take you through offensive and defensive football and try to help you learn more about what you are watching. For the first edition of our "Summer School" series, Chris takes a look at what goes into the 4-3 defensive front.

The 4-3 defense holds a special place in the history of the New York Giants, probably more special than most fans realize.

The 4-3 defense as we have come to know it today has its genesis in Steve Owens' 6-1-4 "Umbrella Defense" from the 1950s. It began to look more like a modern defense when the Giants began flexing the two outermost players on their defensive line back at the snap to help with pass coverage as the forward pass took a more prominent place in NFL offenses.

The Giants became the first team in the league to use a true 4-3 defense as their base defensive front when Tom Landry became their defensive coordinator.

If you want a true measure of the history, think about the Giants Super Bowl 42 victory. They won the championship trophy named after a Giants' offensive coordinator, propelled by a 4-3 defense that was innovated by a former Giants' defensive coordinator, playing with the league's official football, named for the Giants' patriarch.

The 4-3 defense has been the Giants' base defensive front for years now, and going into 2016 it looks like the Giants will once again be lining up in that front more often than not. So with that in mind, let's take a look at the 4-3 defensive front.

Base 4-3 Front

The 4-3 defensive front is named for the number of defensive linemen and linebackers, four and three respectively.

In the original 4-3 defense, the linemen were lined up evenly across the offensive front and rather than the uneven spacing of more modern fronts. Because of this, rather than naming the defensive tackles by their alignment, they are simply the left and right defensive tackles.

In the base 4-3 front there is no nose tackle, the defensive tackles line up directly in front of the offensive guards and the defensive ends are directly in front of the offensive tackles. At the time, 1956, NFL offenses were centered around the running game, and the defensive line's job was to control the offensive line and create a pile.

It was a remarkably successful innovation that helped the Giants win an NFL title, and changed defenses around the league.

However, changes in blocking schemes and the increasing role of the forward pass forced an evolution in the 4-3 defense into the forms we recognize today.

4-3 "Over" Front

In response to evolving offensive schemes, defensive schemes had to evolve as well. One of the driving forces behind the transition to the "Over" and "Under" fronts we know today was Hank Stram, head coach of the AFL Kansas City Chiefs. Stram's innovation seems simple and obvious today, but by shading the defensive tackles to one side of the offensive formation or the other, he was able to pressure offenses and adapt in ways that previous defenses couldn't.

The "Over" front looks fairly similar to Landry's "Base" 4-3. However, by moving the defensive tackles slightly, it created two new positions, and many more options for the defense.

In the "Over" front, the defensive tackle on the weak side of the offense, the side opposite the tight end, moves to line up between the guard and center, becoming the "nose tackle" (NT), while the other defensive tackle slides outward becoming the "under tackle" (DT). By shading the defensive tackles toward the tight end, and toward the strength of the offense, it made it more difficult for run blocking to turn them aside, and provides greater protection to the linebackers.

The two defensive tackles now have different jobs as well. The nose tackle is generally the bigger, more powerful of the two and "2-gaps" or controls both the guard and center. Meanwhile, the other defensive tackle is a bit smaller and more athletic, able to attack the gap between the guard and offensive tackle, and attack into the backfield in passing situations.

Traditionally, as in the diagram above, the strong side linebacker -- SLB or SAM -- lines up on the same side as the tight end, while the weak side linebacker -- WLB or WILL -- lines up on the opposite side. In that case, the expectation is that the bigger SAM (think Devon Kennard) can control or beat the tight end in run defense, while the smaller, faster WILL (J.T. Thomas) can use his agility and athleticism to chase down plays from the back side.

However, some defensive coaches, such as Lovie Smith when he was in Chicago, or Perry Fewell as the Giants' defensive coordinator, did the opposite. They would align the weak side linebacker on the same side as the tight end, while the strong side linebacker would line up on the weak side of the offense. It's a change that was forced by the increasing role of the tight end in NFL passing offenses and the spread of zone counter runs. By swapping the SAM and WILL, it would put the more athletic player on the tight end in coverage, while the bigger linebacker is there for counter runs away from the strength of the offense.

While the "Over" front is the less aggressive of the two modern iterations of the 4-3 defense, that doesn't mean it is necessarily inferior. Both the Chicago Bears and Michigan State Spartans have used the Over front to great effect, fielding excellent defenses over the years.

4-3 "Under" Front

The second, and more aggressive, iteration of the 4-3 defense created by Stram's shading of the defensive tackles has become known as the 4-3 "Under" front.

While the "Over" front shifts the defensive line toward the strength of the offense, keeping a balanced alignment, the "Under" front shifts away from the strength of the offense. The front gets its name from the "Under Tackle", or the defensive tackle who lines up between the offensive guard and tackle on the opposite side of the formation from the tight end.

By moving the defensive tackle to weak side of the offensive formation, the "Under" front is able to stress the offense in ways that the other versions of the 4-3 defense couldn't.

In order to compensate for the shift of the defensive line away from the strength of the offense, an outside linebacker is walked down to the line of scrimmage to help with run support or rush the passer.

The Under front also tends more towards specialization than the Over front, creating specific roles and needs for the positions along the front. Because the Under relies on pressure to disrupt the offense, the Under Tackle and weak-side defensive end (RDE in the above diagram, who also lines up wide of the left tackle's outside shoulder) are generally smaller and faster, better able to create athletic mismatches and penetrate into the backfield. On the other side, the defensive end may tend to be larger and stouter.

The Under front has been making a comeback in the NFL of late, but it has helped to produce some of the most disruptive defensive tackles in league history. Defensive masterminds Tony Dungy and Monte Kiffen used the "Under" front to produce some of the most feared interior rushes in history, with Warren Sapp in Tampa Bay, and John Randle in Minnesota. Pete Carroll took the concepts of the 4-3 Under, learned from Monte Kiffen when the two were on the Minnesota Vikings' coaching staff, with him to USC. There he toyed with introducing some 3-4 concepts into the 4-3 Under front, ultimately creating what would become the Seattle Seahawk's infamous "Hybrid" defense, which we'll get into next week.

Final Thoughts

There are far more wrinkles and variations for the 4-3 defense than we could realistically cover here, at least not without creating a long-form essay. The defensive front has been adapted to take advantage players with unique abilities many times, such as Tom Landry's 4-3 "Flex" defense, created to showcase defensive tackle Bob Lilly, or Buddy Ryan's "46" or "Bear" front, which was originally created to take advantage of strong safety Doug Plank's talents.

This has really been an introduction to the 4-3 defense, its history and how it has evolved into the defense we now see called on Sundays. In general, the 4-3 defense relies on the physical abilities of the defensive linemen to "out athlete", and simply beat their offensive counterparts. While there is plenty of room for finesse and schematic skulduggery in the 4-3 defense, at its core, it relies on the four -- or five, depending on the alignment -- men up front winning their match-ups and disrupting the offense.

Those demands on the defensive line are the reason why the Giants have invested high-round draft picks and a considerable amount of salary cap over the years in their defensive line. A 4-3 defense needs a great defensive line, and players with the size, strength, speed, and agility required are difficult to find.

Because it is the Giants' base defense, we are using it as a starting point for our "Summer School" series, taking a look at the nuts and bolts, the white board concepts, of football. Alex Sinclair and I decided to start this series after the response to the looks at how the offense and defense could evolve under Ben McAdoo.

Alex (who, by the by, is responsible for our cover image, so shout out there!) will be back on Friday with something from the offensive side of the ball, while I'll be back next Tuesday with a look at the 3-4 defense.