• Offensive left tackles routinely go high in the NFL draft because of the position's importance on the field.football image by Maria Bell from Fotolia.com
  
  The game of football begins with the offensive and defensive lines. These athletes fight to control the line of scrimmage, the imaginary line marking where the ball rests on the field. One side tries to overpower the other, the defense trying to stop the play and the offense trying to make the play go for as many yards as possible. National Football League teams covet quality lineman, who earn millions of dollars over their professional careers.
  
  

Center

• The center is responsible for listening to the snap count of the quarterback and "hiking" the football back into the quarterback's hands, according to the Football 101 website. He also must take on the defense's nose tackle in run and passing blocking schemes. The center is often considered the leader of the offensive line because he is the first man on the field to touch the football, and all other offensive lineman key off his initial motion.
  
  

Guard

• Two guards flank the center on either side and provide protection for the quarterback on passing downs and block for the running back in non-passing situations. Guards routinely block defensive tackles and linebackers and may have to assist the center in blocking the nose tackle or to help block a defensive end if a tackle is having trouble stopping him. Guards are massive, usually weighing around 300 pounds and standing well over six feet tall. Anyone smaller is considered too small to play the position.
  
  

Tackle

• The tackle is among the most important positions on offense, according to Football 101, especially the left tackle, who must protect a right-handed quarterback's blind side from the defensive pass rush. The position has become particularly important since the career-ending injury to quarterback Joe Theismann in 1985. Right and left tackles are primarily concerned with blocking speed-rushing defensive ends trying to get to the quarterback quickly and stop a play.
  
  

Defensive Lineman

• Defensive linemen are run-stoppers concerned with blocking up the middle of the field so that a running back has no holes to run through to get up the field and advance the football. Defensive lineman are comparable in size to offensive lineman, typically hovering around the 300-pound mark and over six feet tall to avoid getting leveraged to the ground by taller offensive lineman. Defensive lineman can also block up passing lanes by being tall enough that the quarterback can't see over them to find open receivers down the field.
  
  

Defensive End

• Defensive ends are the primary pass rushers of the defensive line. According to the website Football 101, these athletes can be smaller than defensive tackles, ranging from 245 to 260 pounds but still over six feet tall. The primary duty of a defensive end is to tackle the quarterback for a yardage loss while he still has the football in his hands. Doing so is called a "sack." Defensive ends must also seal the edges of rushing lanes to prevent a running back from getting outside the defensive tackle positions during running plays.