A chop block is an illegal block usually committed by an offensive lineman. It is called when an offensive player executes a block on a defensive player below his thigh when he’s already being blocked by another offensive player. 
Chop blocks can be called on any offensive or special team’s play and are illegal no matter where on the field of play they’re committed or their proximity to the ball.
They’ve been illegal in the NCAA since 1980 and were completely removed from the NFL in 2016 due to the high risk of injury they pose to defensive players.
This infraction results in a 15-yard penalty against the offense and the down is repeated. This is five more yards than other common penalties called against offensive linemen such as holding. 
- A chop block is usually called when a defensive player is hit below the thigh when he is already being blocked by another player
- There are several other variations of the rule in which the defender does not necessarily need to be engaged with an offensive player for a chop block to be called
- The NCAA outlawed chop blocks completely in 1980. The NFL slowly added variations of the chop block to their rule book before making all of them illegal in 2016 to better protect defensive players
Interpretations of the Rule
A chop block can be called for reasons other than striking a defender below the thigh after he’s been engaged. There’s also the “reverse chop,” where the two offensive players hit the defender simultaneously or the defender is hit below the thigh first. 
The official NFL rulebook defines a chop block as:
“A Chop Block is a block by the offense in which one offensive player (designated as A1 for purposes of this rule) blocks a defensive player in the area of the thigh or lower while another offensive player (A2) engages that same defensive player above the waist.” 
With the Steelers’ pass rusher already engaged, a second Cardinals player strikes him below the knees.
The NFL rulebook describes six other specific instances where chop blocks are illegal. Four of which can occur on passing plays and two on running plays. The NFL rules use the same A1/A2 designation for these instances too. A1 refers to a blocker engaged below the thigh while A2 is another blocker engaged above the waist.
For passing and kicking plays, a chop block is illegal when: 
1). A1 chops a defensive player while the defensive player is physically engaged above the waist by the blocking attempt of A2.
Even though the Patriots’ defender has beaten the lineman, he’s still engaged with the blocker when the running back strikes him below the waist. It’s a tough position for the running back. The defender is already applying pressure to the quarterback, and waiting any longer to attempt a block could result in a sack.
2). A2 physically engages a defensive player above the waist with a blocking attempt, and A1 chops the defensive player after the contact by A2 has been broken while A2 is still confronting the defensive player.
Contact has just been broken when the second offensive lineman dives toward the defender’s knees. The upright offensive player is still considered to be confronting the pass rusher, making this low block illegal.
3). A1 chops a defensive player while A2 confronts the defensive player in a pass-blocking posture but is not physically engaged with the defensive player (a “lure”).
Before #78 makes contact with #93, he’s hit below the knee by another blocker.
4). A1 blocks a defensive player in the area of the thigh or lower, and A2, simultaneously or immediately after the block by A1, engages the defensive player high (“reverse chop”).
The reverse chop occurs when the defender is hit below the waist first and is then blocked by a second player. In this example, the defender is struck below the waist first. There isn’t a lot of contact by the second blocker, but enough for the official to call it a reverse chop.
Chop blocks are also illegal in these two instances on running plays: 
1). A1 is lined up in the backfield at the snap and subsequently chops a defensive player engaged above the waist by A2.
This is similar to the chop block’s original definition, but this clarifies that running backs cannot throw a block below the waist when the defender is already engaged.
2). A1, an offensive lineman, chops a defensive player after the defensive player has been engaged by A2 (high or low).
On this running play to the left, two offensive players engage a defender both above and below the waist.
When is a Chop Block Legal on a Running Play?
If an offensive player does not initiate the contact or is trying to avoid the defender, no penalty is called. On many running plays, an offensive lineman will bypass a defensive player to move further upfield and block a linebacker or safety. Incidental contact with a defensive player they’re not trying to block isn’t considered “engaging” them, so no chop block should be called.
Number 64 is trying to avoid contact with the Packers’ defender. So even though two offensive players are engaged when the low block occurs, no penalty is called.
Why is a Chop Block Illegal?
Chop blocks are not allowed and are assessed as such a large penalty because of the high risk of injury to defensive players. 
But the chop block hasn’t always been illegal. For many decades, there was no rule forbidding the chop block at either the NFL or NCAA level. After a rash of injuries, the NCAA made it illegal in 1980. 
The NFL moved slower to outlaw the chop block and it wasn’t made completely illegal until 2016. 
Before 2016, offenses could use the chop block to open wide running lanes for running backs, removing defenders from the play entirely and freeing up blockers to move downfield quicker. 
Chop Block vs. Clipping
Clipping is closely related to the chop block and is called when a defender is blocked below the knees from behind. 
Like a chop block, clipping is illegal because of the potential danger and long-term injuries that can be inflicted when performing this type of block. Because of the risk to the player, clipping also carries a penalty of 15 yards. 
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