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How Glenn Dorsey Fits into the San Francisco 49ers' Defense

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COMMENTARY | The San Francisco 49ers recently gave a two-year, $6 million contract to defensive tackle Glenn Dorsey. Many media critics questioned this signing. Why would the 49ers, who run a 3-4 base defense, sign Dorsey, who does not fit in a 3-4?

The answer is simple: The 49ers don't run 3-4 defense. It may look like they do, but they don't.

In a 3-4, the three down lineman line up in specific spots. The nose tackle lines up face-to-face with the center and the two defensive ends line up face-to-face with the two offensive tackles. The guards are left uncovered. The 49ers do not line up like this.

The 49ers primarily use a one-gap scheme they call an "Under" front. They put their down lineman in gaps like a 4-3 defense, not face-to-face with offensive linemen. The right "defensive end," Justin Smith, lines up in the weak side B gap. (The A gaps are on either side of the center, the B gaps are between the guards and the tackles and the C gaps are between the tackles and the tight ends). When the ball is snapped, Smith rushes around the left shoulder of the left guard, just like a 4-3 defensive tackle does. This is the same position Dorsey played in college.

Aldon Smith lines up in a two-point stance to the right of Justin Smith. The two-point stance is what gives the 49ers' defense the illusion of being a 3-4. Aldon Smith looks like a linebacker because he is standing up. But he's not a linebacker. When the ball is snapped, Aldon rushes around the edge against the left tackle, just like a 4-3 defensive end.

The left defensive end, Ray McDonald, lines up on the outside shoulder of the right tackle and covers the strong side C gap. Dorsey will not play this position for the 49ers. He doesn't have the body type for it. McDonald is 6-3, 290 pounds. Dorsey is shorter and heavier - 6-1, 297 pounds. He needs to match up against centers and guards, not offensive tackles.

The nose tackle lines up in the strong side A gap, between the center and the right guard. This is the position Isaac Sopoaga played for the 49ers the last two seasons, and it's the position the 49ers will ask Dorsey to play at first. The 49ers want Dorsey to have enough quickness at nose tackle to shoot the strong side A gap and sack the quarterback if the center lunges to his left to double team Justin Smith.

If Dorsey succeeds at nose tackle, he will get a chance to back up Justin Smith at "Under tackle," the tackle who lines up in the weak side B gap, the position Dorsey played at LSU. Dorsey also will get a chance to back up Smith at defensive tackle in the 49ers' Nickel and Dime defense, which they use roughly 70 percent of the time.

Justin Smith will be 34 next season, he has one year left on his contract and he's coming off a serious elbow injury. The 49ers need a long-term replacement for him. If Dorsey can be his replacement, the 49ers will do back flips.

But if Dorsey is not ready or capable, they'll settle for him playing only nose tackle, because that's what they need right now, and that's what they signed him to do.

Grant Cohn covers the 49ers daily. He writes the "Inside the 49ers" blog for the Santa Rosa Press Democrat, and has written columns and features for Follow him on Twitter @grantcohn.

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