A horse collar is a penalty in football that is called on the defensive team when a defensive player tackles an offensive player using a horse collar tackle, which is an illegal tackle. A horse collar tackle is a tackle in which a player grabs an opposing player from behind by the back collar of the uniform, under the padding, and pulls the player straight down off of their feet. The horse collar tackle was banned in 2005 by the NFL, deemed the Roy Williams Rule, as the safety was known for using the tackle that caused three significant injuries. This rule is in place to prevent injuries to the neck and leg areas. This penalty results in a 15 yard loss for the offending team.