Historically, sensors have been simply floor mats that sense the weight of a user, one in the area immediately in front of the door (the approach side) and one in the area behind it (the swing side). The approach side mat is often used as a trigger sensor. The swing side mat prevents the door from starting to open as long as some other person is detected in the swing area; once the door starts opening, this mat is ignored, as it will sense the user going through.