They are at the core of every defensive coordinator's true worth. They are the game's substitution defenses and they represent the "chess" match that is played out by offensive and defensive coordinators in NFL stadiums across the country.

You know the terminology: nickel, dime, quarter, 44 and goal-line. And you may even know exactly what they are. But knowing when to use them isn't nearly as difficult a task as identifying the corresponding offensive personnel.

Here's an example. It's third-and-eight and the play clock is ticking down. All of a sudden three offensive players rush onto the field in a tight group. Who are they? That question must be answered before you can order your defensive sub package.

Get it? Good. We'll get back to that third-and-eight play a little later. First, let's explore the sub packages available to us.

Nickel—It's the father of all substitution defenses. The "nickel defense" can be traced back to the day of Johnny Unitas. It is clearly an advance made necessary by the trend to a more wide-open game that evolved in the 1960s. In "nickel," a fifth defensive back is inserted and a linebacker is withdrawn, leaving a 4-2-5 alignment. The "nickel" back is usually the team's third-best cover man, which is usually the team's third cornerback. "Nickel" is used when the offense inserts a third wide receiver.

Dime—A sixth defensive back joins one linebacker, usually the middle linebacker, and four down linemen. There's no significance to the name "dime" other than it is the next denomination of currency available. When is the "dime defense" used? When down and distance makes it nearly certain a pass will be ordered. "Dime" is used in that down-and-distance circumstance against three and four-wide receiver sets.

Quarter or Penny—The offense has gone five-wide, which means the backfield is empty and it's going to be a pass. Get the linebackers off the field and put in three more defensive backs. Four down, seven in coverage; this is pass-offense against pass-defense and a lot of defensive coordinators will seize the opportunity to blitz a couple of their defensive backs.

Forty-four or Elephant—Now let's swing all the way to the other end of the offensive spectrum. These guys like to run the ball. They want to shove it down your throat, so it's time for one of the little guys to retreat to the safety of the sideline and replace him with an extra linebacker. Four down linemen, four linebackers, one cornerback and two safeties; it's the counter defense to a two-tight end, two-running back offensive set. Stand back! Men at work!

Goal-line—Stopping the run has become most serious now. The offense is still in two tight ends and two running backs, but the field has shrunk and the defense doesn't have another yard to spare. This calls for more beef; six down linemen, two linebackers, a cornerback and two safeties. This is maximum run-defense with the bare minimum in pass-coverage capability.

"You want to match the offensive personnel. Put the defenders on the field that match the offensive grouping," Jaguars defensive coordinator Mike Smith said of the intent of substitution defenses. "It becomes a chess game. They put out three wide receivers. How are you going to respond; 'nickel' or 'dime'? Down and distance will dictate that."

All right, let's go back to that third-and-eight play. The down and distance gives you a pretty good idea a pass will be ordered, so you can rule out your "44" and "goal-line" sub packages. You're going to be in "nickel," "dime" or "quarter," but which one? Obviously, the personnel the offense sends onto the field will dictate your reaction.

You and one of your defensive assistants are responsible for identifying the new offensive personnel, and here they come. There are three of them; two wide receivers and a third-down back. You studied your opponent on film all week and you know this personnel grouping. You even have a name for it, and just as you expect, out of the huddle go the running back, fullback and tight end.

Hey, this is easy stuff. The offense is going four-wide with a running back whose primary skill is as a pass-catcher. The offense is pass-heavy and you call down to your man on the sideline. "Dime," you say, and two defensive backs immediately rush onto the field and two linebackers rush off.

Way to go! You did a great job.

Hold on, though. It gets a little tougher. Knowing what sub defense to use is easy. There are only a few that apply to the specific down and distance. If the offense makes its substitutions easy to identify and digest, you'll never make a mistake. But what if they try to deceive you?

Let's go back to that third-and-eight play again. Here come those three guys, but this time two of them stop and turn around and head back to the sideline, and just as they're doing that they are passed by two more offensive players rushing onto the field. Get the point?

The "chess" match is such that first the offense substitutes, then you react. To minimize that reaction, the offense is going to do everything possible to compromise and limit your identification time. They'll run guys on at the same time or stagger them, and the officials will allow it, as long as there aren't more than 11 players in the huddle at any one time.

"The substitution packages are the thing we spend the most time on, making sure we put the best matchup out there on the field," Smith said.

One of the proud moments for Smith and head coach Jack Del Rio last season was the Jaguars defensive staff's performance against Tampa Bay. Bucs head coach Jon Gruden is a master of substitution offense.

"I thought in the Tampa game we were able to match Jon. When you're playing a team like Tampa and a coach like Jon Gruden, you have to be on the top of your game," Smith said.

Smith believes the Jaguars can become even more effective in playing the sub defenses this year. "We've upgraded our personnel but, more so, the flexibility is going to come from the familiarity our players have with our scheme. That's going to allow us to grow it," he said.

"I still think it comes down to players making plays. You're trying to put the defenders in the best matchup situations. You don't want a linebacker on a wide receiver. It's not a good matchup," Smith added, suggesting the science of defensive strategy isn't as difficult as the average fan might believe it is.

And it really isn't, provided the discipline has been built in during the week. Players have to know exactly what to do and they must react on a second's notice. The coaching staff must be organized so that when the orders come down from the coaches' booth, the chain of command is understood and respected without hesitation.

In the case of the defensive coordinator, the real genius lies in the hours of painstaking study. You must know the opposing offense as you know your own defense. It's not enough to know he's a tight end. Is he a pass-catching tight end or a blocking tight end? Reaction time must be knee-jerk.

So, do you think you have what it takes? Well, here are some down-and-distance/offensive formation combinations. You call the defense.

Third-and-three/one back, two tight ends, two wide receivers.

Second-and-six/two backs and three wide receivers.

Third-and-10/four wide receivers and one tight end.

Second-and-10/five wide receivers.

Third-and-six/one back, three wide receivers and one tight end.

First-and-goal inside the five-yard line/two backs, two tight ends, one wide receiver.