The 3-4 defense has been around college football about as long as the modern-day helmet.
Sports equipment company Riddell started marketing its plastic suspension helmet in 1946, shortly before legendary Oklahoma coach Bud Wilkinson devised his innovative "Okie" defense. Wilkinson's defense -- which actually began as a 5-4 alignment before morphing into a 3-4 -- helped the Sooners win 14 league and three national championships, as well as an NCAA-record 47 consecutive games from 1953 to '57.
Former New England Patriots coach Chuck Fairbanks is credited with introducing the 3-4 alignment to the NFL, and the 1972 Miami Dolphins were the first team playing a 3-4 defense to win a Super Bowl.
Over time, the 3-4 gained prevalence in the NFL -- nearly every team has employed it at some point in recent years -- but in the past two decades, the defensive scheme became more of an oddity in college football. With most teams operating run-oriented offenses in college, defensive coordinators preferred a 4-3 scheme to have four linemen filling running lanes.
But heading into the 2010 season, more than a dozen teams will use the 3-4 as their base defense, and a few more teams plan to sprinkle odd-man fronts into their game plans. Georgia, Georgia Tech, Houston, Notre Dame, Stanford, Texas A&M and Texas Tech are switching their base defense to a 3-4 this season, and Alabama, Air Force, Army, Navy, BYU, California and SMU will continue to have three down linemen and four linebackers on the field in 2010.
Coaches and their defensive coordinators offer myriad reasons for the sudden resurgence of the 3-4. With four linebackers, defenses can put more speed on the field to attack spread offenses. Also, coaches say linebackers are more bountiful than defensive linemen on the recruiting trail.
"I think football is cyclical," new Texas A&M defensive coordinator Tim DeRuyter said. "Everyone is trying to stay ahead of the curve on things."
And it certainly doesn't hurt that teams such as defending BCS national champion Alabama have enjoyed so much success while running a 3-4.
"I think if somebody does something and wins with it, wins on a consistent basis, I think more and more people will gravitate to that style," Georgia coach Mark Richt said.
Richt was looking for a solution to his team's defensive woes after the Bulldogs finished 38th in total defense (339.4 yards per game) and 63rd in scoring defense (25.9 points) in 2009. He fired defensive coordinator Willie Martinez and two other defensive assistants, targeting two Nick Saban disciples as their replacements: Alabama defensive coordinator Kirby Smart and Dallas Cowboys assistant Todd Grantham, who worked with Saban at Michigan State.
Smart, a former Georgia defensive back, turned down an offer to return to his alma mater. Richt hired Grantham as his new defensive architect shortly thereafter.
"The 3-4 gives you the ability to adjust, to stay balanced and to adapt," Grantham said. "If I'm playing a 4-3 defense, those four guys with their hands in the dirt are going to rush the quarterback 90 percent of the time. I think the 3-4 gives you versatility, and it's a little tougher for the offenses to predict where the pressure is coming from. It's harder for the offense to prepare."
But teams can't switch from a 4-3 to a 3-4 overnight. Shortly after Grantham accepted the Georgia job, he had the team's graduate assistants produce video clips of 30 plays for each returning player on defense. Grantham tried to use the film to figure out where Georgia's returning players fit into his scheme.
"I think any time you change schemes, there are going to be some differences," Grantham said. "There are going to be some things you have to work through."
Perhaps the most important cog in a 3-4 defense is the nose guard. He needs to be big enough and strong enough to take on double-team blocks, and he can't allow guards to reach the linebackers behind him. Alabama's defense was among the country's best in each of the past two seasons because since-departed nose guard Terrence Cody was so hard to block.
A defensive end's role also changes significantly in a 3-4. In a 4-3 alignment, defensive ends line up outside a tight end or offensive tackle, where they hope to use their speed and athleticism to beat blocks. Most of the time, an end's sole responsibility is to rush the quarterback and collapse his pocket. In a 3-4, an end's most important job is to control gaps and beat double-team blocks to push the pocket.
Because their sole responsibility often is to take on blocks, playing nose guard and defensive end in a 3-4 is considered somewhat unglamorous. Ends and tackles won't produce the eye-popping stats they might get playing in a 4-3. More than anything else, their job is to free up linebackers to make plays.
The weakside linebacker usually is the premier pass-rusher in a 3-4. But he also has to be big enough and strong enough to beat blocks from offensive tackles and running backs. A strongside linebacker often is asked to drop into space and cover tight ends and running backs on passing plays. The two inside linebackers are supposed to find holes in the line and make plays in the running game or pressure the quarterback.
Grantham says the biggest advantage of a 3-4 scheme is its unpredictability. Any of the four linebackers can blitz on a given play, or one of the outside linebackers can move to the line of scrimmage as a stand-up end.
"If you're multiple in what you're doing, you can bring any of the four linebackers at any time," Grantham said. "They have to be accounted for in protection on every play. They don't know which guy is going to be coming."
That unpredictability also tends to lead to more turnovers. Under DeRuyter's guidance, Air Force's defense led Football Bowl Subdivision teams in turnover margin last season. Alabama was fourth. It's part of the reason Texas A&M coach Mike Sherman added DeRuyter to his staff this offseason and handed him the reins to the Aggies' defense.
"I want us to be aggressive on defense," Sherman said. "I think we have an offense that should be able to move the ball if everybody does their job, and I want to be able to force takeaways. I think the 3-4 structure can present issues on where the pressure is coming from, particularly against the spread offenses. You can drop bait; you can bring guys from different sides. So really the offense cannot predict where the pressure is coming from."
Finding the right players to fill roles in a 3-4 often is easier than finding 330-pound tackles, which are necessities in a 4-3. The availability of capable linebackers in recruiting is one of the reasons so many teams are moving to a 3-4.
"I think it's probably a number of reasons," said Michigan State coach Mark Dantonio, whose team will experiment with a 3-4 this season. "I think they probably want to get the best players on the field and it's easier to find linebackers. Defensive tackles are tough to find. Defensive tackles are a premium [position]. When you do find them, everybody wants them. He may have 30 scholarship offers."
The service academies were among the first teams to return to a 3-4 scheme. They often have undersize defensive linemen, which leaves them at a disadvantage against teams from BCS conferences.
"I think you can find the body types to play the outside linebacker position and they can play earlier in their careers," Grantham said. "If he has to play defensive end in the 4-3, you have to develop a guy strengthwise before he can contribute to your team. If a guy is playing outside linebacker, he plays in space, so he can rely on his athleticism and speed more."
New Notre Dame coach Brian Kelly switched to a 3-4 defense in his last season as Cincinnati's coach in 2009. He brought former Bearcats defensive coordinator Bob Diaco with him to Notre Dame.
Even though a 3-4 defense might be more adept at putting pressure on the quarterback, Kelly says a defense's No. 1 priority still has to be stopping the run. And 3-4 teams will have to do it with one fewer lineman.
"I think what we need to see more than anything else from our defense is the ability to stop the run on a consistent basis," Kelly said. "Your inside [line]backers have got to be able to press guards; they've got to do a great job there. I know we can bring pressure from the edge with our athletes. We've got to be able to hold up inside out."
In the end, if a player doesn't carry out his assignments and tackle well, it really doesn't matter what type of scheme a defense uses.
"I do believe you can win running just about any system," Richt said. "You can play great defense in a 3-4. You can play great defense in a 4-3. You can play a great defense in a 3-5, however you want to align it. I think the thing that's important is your personnel, how well they understand what you're doing and how well they execute what you're doing."
Mark Schlabach covers college sports for ESPN.com. He co-authored Bobby Bowden's memoir, "Called To Coach," which was published by Simon & Schuster. The book will be available in bookstores Aug. 24 and can be preordered here. You can contact Mark at email@example.com.