(May 11)—It didn't take Mike Smith long to identify what would be the strength of his Jaguars defense. Defensive tackles Marcus Stroud and John Henderson stood out as the best of what was left from the 20th-ranked defense the Tom Coughlin regime left behind, and that was fine with Smith and Jack Del Rio because the defensive system they embraced is built on stopping the run.

Those conclusions were reached shortly after Del Rio replaced Coughlin as Jaguars head coach and Smith became the team's new defensive coordinator. As the new coaching staff's offices were being remodeled, personnel evaluations were also underway, and reviews were mixed. Stroud and Henderson represented a couple of cornerstones, but there were gaps in between.

Del Rio and Smith decided they would remain basic in their scheme and work hardest on player development and the application of the talent that was available to them. In short, they knew they could stop the run but rushing the passer and defending the pass would be more difficult.

"What's more important, the scheme or the players? The players. You want your scheme to fit your players," Smith said of what the Jaguars defensive staff did last season.

They used Stroud and Henderson much as the Ravens used Tony Siragusa and Sam Adams in 2000; to dominate the middle of the line of scrimmage and force double-teams that keep blockers off the linebackers and allow them to penetrate, disrupt and, ultimately, make the tackle.

In the secondary, however, the Jaguars were not able to play Ravens-like defense. The Jaguars lacked the pass-rush to play pressure coverages. No gambling for the Jaguars last season; they lacked the pass-rush to limit the quarterback's time, or the man-to-man coverage skills to add blitzers to the rush.

What to do? The answer was simple.

Behind the 4-3 defensive front that is the foundation of Del Rio's and Smith's shared defensive philosophy, the Jaguars played the cautious "cover two" and "cover three" pass-defense schemes that are the most popular coverages in the game today.

You've heard the term: "Cover two." Everybody knows it's Tony Dungy's defense, but "cover two" was being played in the NFL long before Dungy was even a player. It's as simple as they come: Two half-field defenders and five guys underneath.

In "cover two," the two safeties are positioned deep and just outside the hash marks. Each safety is responsible for his deep half of the field. Meanwhile, the underneath coverage consists of two corners responsible for jamming the wide receivers at the line of scrimmage, then falling off into zone coverage in the flat areas. At the same time, the three linebackers have dropped about 10 yards, forming a wall of defenders across the field.

"The theory is that it's a re-route defense and you want to disrupt the receivers running downfield," Smith said. "The cornerbacks are responsible for jamming the receiver and forcing him to the inside. It allows you to force the ball to get checked down (thrown short). It takes away the deep ball."

Sounds great, right? Yeah, but it all starts with stopping the run with your down linemen because you don't have that eighth man in the box to help against the run. "Cover two" is a pass-defense for teams with strong defensive linemen who don't require help, and that's just what Del Rio and Smith decided they had. The Jaguars' strength was up front.

So, how did the Jaguars so dramatically improve their defensive ranking last year; from 20th to sixth overall? By improving from 25th to second in run-defense. That's how.

This season, the Jaguars will be more sophisticated schematically. Smith believes offseason personnel upgrades will deepen the team's scheme. Last year, personnel limitations restricted the Jaguars to "cover three" as its changeup defense.

What's "cover three?" Some might say it's a great high school defense.

In "cover three," one safety joins the two cornerbacks to form a three-deep alignment. The other safety, usually the strong safety, joins the three linebackers to create four underneath. In other words, "cover three" is two picket fences behind a four-man front. What it lacks in that extra underneath pass-defender, "cover three" provides in the way of an extra run-defender or eighth man in the box. It's a good defense against the run and the pass, but it's very conservative.

Want something risky? Here are two you might like.

"Cover one" and "cover zero" are man-to-man pass-defenses. They are intense pressure coverage schemes.

In "cover one," a fifth pass-rusher is added and there's a safety stationed in the middle of the field to help ease the cornerbacks' deep burden. In "cover zero," a sixth rusher is added and the safety is subtracted from the middle of the field. In "cover zero," the rush either has to get "home" or you better have Deion Sanders and Rod Woodson as your corners.

You don't like that kind of risk? That's to be understood, and if you're really conservative in nature, "cover four" is your baby.

"Cover four" is four deep and three underneath. It is the most basic of all zone pass-defenses and is affectionately known as "quarters."

The bottom line is that all of the conservative pass-defenses can be effective, provided you can stop the run with your down linemen. The more defenders who may be committed to the pass, the better the pass-defense will be. Del Rio's and Smith's defensive scheme last year was built on that up-front principle.

In their 4-3 alignment, the defensive tackles are responsible for jamming the inside running lanes by occupying blockers. With that having been accomplished, the three linebackers are responsible for "scraping" to the ball and making the tackle. The defensive ends set the edge of the defense.

The other popular defensive front used in today's game is the 3-4, which employs a nose tackle, two defensive ends and four linebackers. The nose tackle's job is to defeat the center and force a double-team from one of the guards. The defensive ends stuff the OT/TE gap and hold the point. In the 3-4, the defensive linemen are not responsible for disrupting the play. That task falls on the linebackers, who are usually fast and aggressive types capable of forcing the action and making the play. The chief advantage of the 3-4 is that it causes greater confusion in the offense's blocking scheme because the 3-4 can alter its alignment so dramatically.

These are the basic defensive formations used in today's pro game. They represent only a fraction of all defenses, which include "nickel," "dime" and other substitution packages that will be the subject of a future "Defense 101" installment.

Now, here are some basic defensive terms you might find useful:

• Stunt—Change in the charge of a defensive lineman, who lines up in one gap and then charges through another.

• Stem—The pre-snap movement of a defensive lineman from one position to another.

• Dog—A fifth rusher.

• Games—Looping or twisting by two defensive linemen working in tandem during a pass-rush.

• Squat corner—A "cover two" cornerback; one who "squats" or "sits down" in the flat area. Right cornerbacks are often used in that capacity.

• Mike—Middle linebacker in 4-3 or strongside inside linebacker in 3-4.

• Mo—Weakside inside linebacker in 3-4.

• Will—Weakside outside linebacker in 3-4 or 4-3; usually lined up on side opposite tight end.

• Sam—Strongside outside linebacker in 3-4 or 4-3; usually lined up on tight end side.