In a careful, conservative game of curling, teams take out guard rocks as quickly as they are placed. This leads to a game free of clutter, but it also makes the game predictable. The first team to play always places a guard rock or a rock close to or in the house, and the second team always takes it out of play by hitting it hard at an angle, or 'peeling' it. The score in such a game will be low, with many blank ends and a few single points as the team with last rock manages to leave it in the house. Wanting a more aggressive, spectator-friendly game, the World Curling Federation added section R6 to its rulebook, the Free Guard Zone Rule.
Section R6 has two parts. The first defines the boundaries of the free guard zone as the part of the curling sheet between the tee line and the hog line at the playing end, excluding the house. Since stones which stop short of the hog line are usually removed and stones which go into the house can potentially score, the in-between free guard zone is where teams usually set up their guard stones. Once a guard stone is set up and stays up, a second stone is sent into the house behind it by curling the second stone completely around the guard stone. Ideally, the guard stone now stands in a nearly straight line in front of the potentially scoring stone, protecting it from a takeout.
The second part of Section R6 states that no rock in the free guard zone can be removed from play by the opposition until four rocks have been thrown in that end, two by each team. Should this happen accidentally, the delivered stone is removed from play, and all other stones are returned to their original positions. This forces teams to play with at least one and potentially as many as four rocks in front of the house, greatly increasing the tactical difficulty of play.
It is as important to understand what the Free Guard Zone Rule does not say as what it says. It does not forbid an opposition team from hitting stones in the free guard zone, so long as they do not go out of play. It also does not forbid a team from taking its own stones in the free guard zone out of play. These two deliberate loopholes have given rise to some very difficult tactics such as Kevin Martin's 'tick' game, where the guard stone is hit hard enough to move it away from being effective as a guard stone or usable for anything else, but leaving it still in play.
Because the Free Guard Zone Rule arose from a suggestion made by Russ Howard in 1991 during a Moncton, New Brunswick, tournament, it is sometimes known as Howard's Rule, or as the Moncton Rule. The rule was used for the first time during this tournament, but in a different form. Instead of limiting the out-of-play rule only to those rocks in the free guard zone, it stated that no rock could be removed from play until the fifth stone was thrown. For a while, several versions of the rule were in use simultaneously at different levels of play. The Modified Moncton Rule or Three-Rock Rule, which limited the number of rocks at three instead of four, was briefly in use in Canada until the Canadian Curling Association finally adopted the World Curling Federation's Free Guard Zone Rule in its current form.