This writeup details a few of the most common penalties in NFL football, some tips on how to spot them, and the associated referee's hand gestures, so that you can perform them at the TV in addition to yelling about how the refs are robbing your home team of yet another victory.

Offsides. When a member of the defense is on the wrong side of the line of scrimmage when the ball is snapped.
Signal: Referee puts hands on hips, arms akimbo.
Offsides is usually called when a defenseman jumps too early, that is, starts rushing the passer a half-second before the ball is snapped. The play is still live, so often a quarterback will attempt a long pass if he knows the defense was offsides, knowing that he can accept the penalty in any case.

Encroachment.
When a member of the defense comes into contact with a member of the offense before the ball is snapped.
Signal: Same as offsides.
This is basically the same as offsides, except the play is whistled dead.

False Start.
When a member of the offense moves before the ball is snapped.
Signal: Referee puts fists in air in front of chest and moves them around each other in circles.
The offense is not permitted to move, so that the defense will know when the ball is snapped, because then everyone will start moving. This penalty stops play, so if you hear a whistle before the snap, and didn't see any encroachment, it's either this or a delay of game penalty.

Holding.
When a member of either team, um, holds an opponent.
Signal: Referee puts fist in air near his face, palm facing inward, and grabs his arm with his other hand, pulling it down.
Linemen are allowed to push and shove, but not to grab their opponent. There's a little holding on every play, but the most blatant holds win a flag. If you see a flag in the backfield before the ball is thrown, it's probably holding on the offense.

Pass Interference.
Through holding, pushing, or shoving, when a defender interferes with the receivers ability to catch the ball, or when the receiver similarly interferes with the defender.
Signal: Ref pantomimes pushing someone who is right in front of him.
This penalty can often be a pretty big judgement call, and given the severity of the penalty, can lead to high tempers all around. Once the receiver is in the act of catching the ball, however, the defender is free to tackle him and cause an incompletion.

Delay of Game.
When the play clock runs out before the offense snaps the ball.
Signal: Ref puts hands on head, elbows out.
After the end of one play, the offense has snap the ball for another play within thirty seconds. Once lined up, they have to snap the ball within ten seconds. Otherwise they get called for delay of game. The TV will helpfully display the play clock on the screen once it gets down to a few seconds.

Intentional Grounding.
When the quarteback standing in the pocket throws a pass to no one.
Signal: Ref puts hands in air to the right of his head, then moves them down and left in a forward-slash motion.
When the quarterback is in the imaginary area in between where the tackles (the guys on the left and right side of the line) lined up, he cannot just throw the ball away to get an incomplete pass. This will result in the intentional grounding penalty. The quarterback must either throw it vaguely near someone (the most popular is over a receiver's head out of bounds) or get out of the pocket and throw it into the dirt. This penalty usually happens when the quarterback is about to get sacked.

Personal Fouls:
Signal: Ref holds out arm and karate chops it with other hand.

Facemask.
When a player grabs another player by the helmet's face mask.
Signal: Ref pantomimes pulling on his imaginary facemask.
Neck injuries are bad. You are not allowed to put your hands anywhere near your opponents face mask. It's five yards for some of such contact, fifteen if you actually grab the mask.

Unnecessary Roughness.
Any rough play excessive of that needed to make an effective tackle.
Signal: ?
Usually called for hitting a guy after he's clearly gone out of bounds, or any other unwarranted aggression.

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