A fourth-dimension roller coaster is a type of steel roller coaster where riders are rotated independently of the track's orientation about a horizontal axis perpendicular to the track. This feature allows riders to experience inversions without the track rotating upside down. Some fourth-dimension roller coasters are controlled, with seats that rotate based on the spacing of two additional rails on the track. Others are free-spinning, where the seats flip primarily due to forces exerted by the ride's elements, offering a different experience during each ride.