[Poohbah]

What is the "21 foot rule"?

That happens to be the distance in which most lethal shootouts take place.

[Redcloak]

What is the "21 foot rule"?

That happens to be the distance in which most lethal shootouts take place.

It's also the distance than an attacker can cover in the time it takes the average police officer to draw and fire his sidearm. Allowing an assailant to close to within 21' presents the possibility that the officer could be injured or killed; even if he manages to fire his weapon and hits his attacker. Within 21', a charging attacker's momentum will carry him to the officer, even if he's been mortally wounded.

[Shooter 2.5]

What is the "21 foot rule"?
That happens to be the distance in which most lethal shootouts take place.

That's not correct. It's a measurement in which a person with a knife is a direct threat to a person with a holstered firearm. I believe Massad Ayoob was one, if not, the first to publish the phrase.

[Abundy]

It is also the minimum distance where you are safe from an edged weapon if your firearm is still secured in your duty holster.

Studies with trained police officers have shown that if a suspect is inside the 21' zone he/she will cut you before you can draw and fire your weapon. The zone increases if the firearm is concealed.