The people in the stripped shirts are the officials. They carry out various tasks such as raising their arms to signal a touchdown or field goal, deciding if a pass was caught in bounds or out-of-bounds, placing the ball on the line of scrimmage for the next play, measuring to see if 10 yards have been covered, and assessing penalties for rule infractions. To call a penalty, an official takes a piece of yellow cloth, called a penalty marker or flag, from his pocket and throws it on the ground. There is then a flag on the play.

Illegal procedure: An offensive linemen moved before the play started or a receiver who was in motion before the play started did not move parallel with the line of scrimmage. Or the quarterback was past the line of scrimmage when he threw a pass. 5 yard penalty.

Ineligible Receiver Down-field: An offensive lineman was too far advanced past the line of scrimmage when the quarterback threw a pass. 5 yard penalty.

Delay of Game: The offensive team took more than 25 seconds to start the play. 5 yard penalty.

Grounding the Ball: The quarter back threw the ball away instead toward a possible receiver because he was about to be sacked. 5 yards and loss of down.

Off-sides: A defensive player moved across the line of scrimmage and either made contact with an offensive player or failed to get back before the play started. 5 yard penalty.

Holding: An offensive lineman used his hands while blocking. 10 yard penalty.

Clipping, Illegal Block, Chop Block: Blocking a player from the back. This can hurt a player because he doesn't know the hit is coming. 10 yards.

Pass Interference: Grabbing or tackling the pass receiver while the pass is in the air. The defender must wait until the ball arrives. However, the defender can try the catch the ball himself, so he may collide with the receiver as they both try to make the catch. If there is interference, the penalty is 15 yards and a first down is granted even if the ten yards has not been made.

Unnecessary Roughness, Roughing the Passer, Roughing the Kicker: Tackling someone after he no longer has the ball or after the play is over. 15 yards.

Facemask: Grabbing the front of a player's helmet during a tackle. 15 yards if flagrant; 5 yards if incidental.

Unsportsman-like conduct: 15 yards.

After a penalty is called, the other team can accept or refuse the penalty. When a penalty is assessed, the ball is brought back to the original line of scrimmage and the penalty is marched off from there. The down is then repeated unless the penalty includes loss of down. That is, the play with the penalty does not count as one of the four allowed to get 10 yards. Thus, if the team with the penalty did poorly on the play, the other team may want to turn down the penalty and accept the play as it was.

If both teams get a penalty on the same play, the play is repeated from the original line of scrimmage.

The maximum penalty is half the distance to the goal line even if a larger penalty would otherwise be assessed.