Below, is an image of the boundary zones for a curling stone. The shaded area represents where a stone can be considered in-play. Additionally, these lines also serve as a basis for where curlers are allowed to stand when their team and their opponent is shooting, which we will learn about in Curling Etiquette.
If any part of a stone crosses the sideline, it is considered to be out-of-play, and is removed immediately. Usually, this does not happen when a stone is being thrown, but could occur. This may happen if the stone is thrown and hits another stone at rest, causing it to reach one of the sidelines.
If a stone fully crosses the backline, it is considered to be out-of-play, and is removed immediately. The stone is in-play as long as any part of it remains in front of the backline, even if it is just a tiny sliver of the stone.
The Hog Lines
After a stone is delivered, it must fully cross the farther hog line to be considered in-play. Even if a tiny sliver of it is in front of the hog line, the stone must be removed from play. There is one exception to this rule. If a stone runs into another stone and cannot fully cross the hog line, it is allowed to remain in-play. This is because if the other stone wasn't there, the thrown stone would have crossed the hog line, and would be in-play.