jammer status cannot be transferred to other skaters, but certain actions (such as being sent to the penalty box) cancel lead jammer status, meaning that the jam has no lead jammer and must continue for the full two-minute period. If the jam is not called off by the lead jammer, it ends after two minutes. If time remains in the period, teams then have 30 seconds to get on the track and line up for the next jam. If the 30-minute period ends while a jam is underway, the jam plays out to its natural conclusion. Blocking A skater may block an opponent to impede their movement or to force them out of bounds. The blocker must be upright, skating counterclockwise, in bounds, and within the engagement zone. Groups of blockers on the same team typically create formations, known as walls, to prevent the opposing blocker from passing. Blocking with hands, elbows, head, and feet is prohibited, as is contact above the shoulders, below mid-thigh, or to the back. Penalties Referees penalize rules violations. A player receiving a penalty is removed from play and must sit in the penalty box for 30 seconds of jam time. If the jam ends during this interval, the player remains in the penalty box during the subsequent jam until the interval ends. While the penalty is being served, the penalized player's team plays short-handed, as in ice hockey. A player "fouls out" of the game on the seventh penalty, and is required to return to the locker room. A "power jam", derived from ice hockey's "power play", refers to a scenario when one team's jammer is sent to the penalty box. In this case, that jammer's team cannot score. If the lead jammer is penalized, no one can prematurely end the jam. It would be pointless to play if neither team could score; thus, both jammers cannot serve a penalty at the same time. If one jammer is sent to the penalty box while the opposing jammer is already serving a penalty, the opposing jammer is released from the penalty box early. The second jammer's penalty is then only as long as the amount of time the first jammer spent in the box. Equipment Players skate on four-wheeled ("quad") roller skates, and are required to wear protective equipment, including a helmet, wrist guards, elbow pads, knee pads, and mouth guards. All current sets of