hog line

In curling, the hog line is a line perpendicular to the direction of play on each end of the sheet, 21 feet from the center of the target (or "button"). When sliding the stone, the skip must release the stone before reaching the hog line, or a penalty is called.

Source: http://www.paulgross.org/mwb2.html

The hog line in curling plays two important roles. If you are the shooter, as you are sliding out and delivering the rock, you must release the rock before it passes over the hog line that is nearest to you. In national and international play special rocks with metal handles and electronics inside are used that will indicate at the hog line whether you took your hand off it in time or not. If not, the rock is taken out of play.

At the other end of the ice, only rocks that have completely passed over that hog line are in play. Any rock that stops on or short of the hog line is taken out of play, unless it stopped there because it hit a rock that is in play.