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On each side of the center are the guards, which are unimaginatively called the left guard and the right guard. These guys have two different possible jobs to do. If the quarterback has called a pass play, where he will try to throw the ball over a mess of very large guys who are all wrestling and hit a speedy receiver in full flight, then the job of the guards is to be large impenetrable walls and absorb any punishment the defensive guys care to put out. The guards are to form a wall with the center that the quarterback can stand behind, unmolested, while the quarterback tries to get the ball to the right person. If the football team spends most of their time passing the ball, the guards will tend to be very big guys, weighing perhaps as much as 325 to 330 pounds. These very heavy guys can't move as fast as somewhat smaller guys, but they can absorb a lot of punishment.
If the quarterback has called a running play, where one of the players will try to run with the football, the guards have their second job. In this case their job is to bull rush whoever is ahead of them and create a hole in the defense for the runner. The guards which are best at bullying a hole in the defense are just a bit smaller, perhaps 300 to 310 pounds, and a bit faster and more athletic. It's a very different job to run forwards a few steps while pushing a 320 pound defender than to stand more or less in one place and take whatever the 320 pound defender dishes out without moving or flinching.
We call these two jobs pass blocking and run blocking. Pass blocking is the job of standing still and not letting anyone through or by. Run blocking is the job of leaping out and trying to put a defender on his butt.
If the quarterback is a relatively immobile guy, then the guards must be very large "wide body" models to be able to stand and absorb punishment, as the quarterback is not going to succeed if he has to run for his life. If the quarterback is a faster guy, then the team can use slightly slimmer, faster, more athletic guards. These guards will be better at run blocking, but will be just a little less reliable in pass blocking. However, if someone gets past the guards the assumption is that the mobile quarterback can run a few steps and get away for a couple of seconds.
Most teams think that it's relatively easy to replace guards, so like centers these are not the highest paid guys on the team. You would expect a guard to be paid $1M to $3M per year.
On the outside of the guards are the two tackles, again called the left and right tackles. These guys have a very hard job to do. The tackles will often be the biggest guys on the football team - 6'6" or even more, 330 to 350 pounds. The defense is sometimes going to line up 320 pounds guys to try to bowl over the tackles. Other times they will line up a 250 pound speed rusher who will try to push off and run past the tackle. Other times they will line up a 180 pounds safety or cornerback who has near Olympic sprinting speed, and he will try to run around the tackle before he can move. The tackle gets no sympathy, it's his job to keep anyone from getting around the end of the line and to the quarterback, no matter how big and strong or small and fast.
Tackles are chosen to have very long strong arms so that they can reach out and stop guys. They must have very fast feet so that they can quickly slide left, right, or backwards as necessary to protect their quarterback.
If the quarterback calls a running play, the tackles will have to run forwards a couple of steps and grab a smaller, lighter, faster guy and keep him away from the guy running with the football. The right tackle generally has the tight end next to him. Because of the extra blocker, this is called the strong side of the line. The defense will usually put their largest and best run stopping end against the right tackle, so he has to be the stronger of the two tackles.
The left tackle protects the blind side of the quarterback - since most quarterbacks are right handed, they tend to look towards their right a bit and away from their left. The quarterbacks are relying on their left tackle to protect them from pass rushers. The defense will generally put their fastest and best pass rushed up against the left tackle, so this guy has to be extra fast and exceedingly dependable. If the left tackle misses his block, the quarterback is going to get hit very hard from behind by a 250 pound speed rusher, and very likely drop the football. The left tackle often "plays in space," meaning the edge rusher will start from a bit outside and to the tackle's left, and then try to run around either side of the tackle. The left tackle will most often have to drop back away from the guard, and then be prepared to block the pass rusher from running around either side of him. This requires that the left tackle has particularly quick feet, so that he can slide side to side a couple feet in almost no time.
An outstanding offensive tackle is a rare gem, and is paid as much as $10M per year. Tackles are among the highest paid players on the football team.