Some high pressure systems maintain a constant distance from the vehicle to provide more consistent results. Separate high pressure top, wheel, and/or side blasters may be present in place of or supplementary to a high pressure arch. Following the idle zone and/or high pressure arch (if applicable) is the friction zone. In the friction zone, a variety of brushes are used to clean all of the vehicle's surfaces. Wrap-around brushes (wraps) clean the front, sides, and rear of a car. A set of wraps consists of two offset brushes which are attached to overhead arms which are generally mounted over the center of the tunnel. Some wrap-around brush designs use jointed side-mounted arms to increase the brush's front and rear contact time. Wraps typically rotate with the vehicle's direction of travel. Some wrap-around brushes are capable of switching direction while cleaning the front of a car to prevent customers' license plates from being bent. Wraps are supplemented by side brushes. Side brushes rotate against the vehicle's direction of travel to increase cleaning power. Top brushes are rotating brush assemblies which clean the vehicle's top surfaces. A top brush can face either the entrance or exit of the tunnel to assist in cleaning the car's front or rear. Some specialized top brushes are designed to achieve more contact time with the front or rear of vehicles. To prevent damage, a top brush can be programmed to retract if an ultrasonic pick-up truck bed detection system detects a pick-up truck with an open bed. In many car washes, top brushes are either supplemented by or replaced with mitters. Mitters are ribbon-like components which suspend cloth strips or sheets over the tunnel while utilizing motion to increase friction against the car's surface. The friction zone may also include specialized front grill and rear brushes. Front grill brushes clean the front, and in many cases, the sides of vehicles. They often appear as side washers with an unusually long and bent arm. Instead of bent arms, some front grill brushes use jointed arms. Rear brushes appear as a specialized tall side washers with long arms that face the entrance of the tunnel. Once a car's mirror passes a rear brush, the brush makes contact with the vehicle's side, where it then continues to engage until the assembly cleans the rear of the car. Additional high pressure components may also be present in the main friction zone. Older automatic washes - a majority of which were built prior to 1980 - used to use brushes with soft nylon bristles, which tended to leave a nylon deposit in the shape of a bristle, called brush marks, on the vehicle's paint.