In American football, referees can signal when the defense either makes an illegal play, warranting a penalty, or scores. Only a few signals are specific to the defense in a football game, but the referees know these signals well.
Illegal contact.
![Illegal contact.](https://www.dummies.com/wp-content/uploads/283446.image0.jpg)
The referee extends his arm and an open hand forward to signal that illegal contact was made.
Illegal crackback block.
![Illegal crackback block.](https://www.dummies.com/wp-content/uploads/283447.image1.jpg)
The referee strikes with an open right hand around the middle of his right thigh, preceded by a personal foul signal, to signal an illegal crackback block.
Illegal cut block.
![Illegal cut block.](https://www.dummies.com/wp-content/uploads/283448.image2.jpg)
From the side, the referee uses both hands to strike the front of his thighs to signal that a player made an illegal cut block. When he uses one hand to strike the front of his thigh, preceded by a personal foul signal, he means that an illegal block below the waist occurred. When he uses both hands to strike the sides of his thighs, preceded by a personal foul signal, he means that an illegal chop block occurred. Finally, when he uses one hand to strike the back of his calf, preceded by a personal foul signal, he means that an illegal clipping penalty occurred.
Interference.
![Interference.](https://www.dummies.com/wp-content/uploads/283449.image3.jpg)
The referee, with open hands vertical to the ground, extends his arms forward from his shoulders to signify pass interference or interference of a fair catch of a punted ball.
Safety.
![Safety.](https://www.dummies.com/wp-content/uploads/283450.image4.jpg)
The referee puts his palms together above his head to show that the defensive team scored a safety. Look for this signal whenever the offense is operating near its own goal line with the quarterback and running backs actually lined up in the end zone.
Uncatchable pass.
![Uncatchable pass.](https://www.dummies.com/wp-content/uploads/283451.image5.jpg)
The referee holds the palm of his right hand parallel to the ground and moves it back and forth above his head to signal that a forward pass was uncatchable and that no penalty should be called.