3-4 or 4-3 Under?

What’s the correct name for the 49ers’ base defense: 3-4 or 4-3 Under?

If you’ve never heard of the 4-3 Under, click here to read a detailed explanation of it.

The article says George Seifert used the 4-3 Under when he was the defensive coordinator for the 49ers. His defense looked like a 3-4 because the weakside defensive end rushed from a standing position, a two-point stance.

Here’s a description of the 4-3 Under alignment from the above article:

“As you can see, in a basic 4-3 Under, the SAM linebacker is lined up to the outside shoulder of the tight end off the line of scrimmage a yard or two and is responsible for the D gap (to the outside of the tight end). He’s also responsible for running in pass coverage from time to time. The strongside defensive end is lined up to the outside shoulder of the tackle, in a 5-technique alignment, and is responsible for the C gap (to the right of the tackle). The strongside defensive tackle is usually lined up shading the center in a 1-technique alignment and is responsible for the strongside A gap. The weakside defensive tackle is lined up in a 3-technique alignment off the weakside guard and is responsible for the weakside B gap in front of him. The weakside defensive end is lined up to the outside shoulder of the weakside offensive tackle and is responsible for the C gap on his side. This leaves the strongside B gap and the weakside A gap open. These are the responsibilities of the MIKE and WILL linebackers.”

Fangio’s defense aligns and functions exactly like this, but he makes a couple of small tweaks.

Seifert had his standing defensive end, or “Elephant,” move around the formation depending on where the offense lined up the tight end. If the tight end went in motion, the Elephant would run over to the other side of the formation to rush the quarterback uninhibited. The strong side linebacker also ran around the formation, following the tight end and covering him one-on-one.

As you know, Fangio’s 49er defense does not feature a stand up DE and a SAM linebacker who run around the formation. That’s 4-3 personnel. 4-3 personnel is rigid, players playing one position. 3-4 personnel is more flexible because most players play two positions: Justin Smith and Ray McDonald can play three and five-technique. Patrick Willis and NaVorro Bowman can play middle and weakside linebacker.

The 49ers have 3-4 personnel, which they use to make the 4-3 Under scheme more flexible and efficient. Let me explain.

Fangio has two defensive end/outside linebacker hybrids – Aldon Smith and Ahmad Brooks. They both can rush and drop into coverage, so they don’t rove around the formation. Smith always lines up on the defense’s right and Brooks always lines up on the defense’s left. If the tight end is on the defenses’s right, Brooks is the stand up DE and Smith is the SAM. If the tight end lines up on the other side, Smith is the stand up DE and Brooks is the SAM.

The 49ers use their 3-4 personnel to tweak the 4-3 Under in one more way: Smith and Brooks both are better at rushing than dropping into coverage, so Fangio rarely asks either player to cover a tight end one-on-one. Instead, he asks them to cover a short zone when they drop. Inside linebackers Patrick Willis and NaVorro Bowman, two of the best covering linebackers in the NFL – cover the tight ends.

Do these minor tweaks make the 49ers defense a 3-4, or is it still a 4-3 Under? How would you classify the 49ers defense? Be as specific as you can be.