a doorway at the bottom of the stairs or causes an obstruction, a hinge can be fitted so the end of the rail can be folded back out of the way when not in use. Curved rails are made from materials such as steel or aluminum and come in various cross-sectional shapes according to the designer. Individual designs vary a lot and probably the key criterion is to make the curves with the smallest radius possible so they will wrap tightly around objects such as newel posts. The sections of curved rails are usually packaged well to prevent damage in transit and are unwrapped and assembled on site. Rails for wheelchair platform stair lifts may be secured to walls in addition to the step fixings. Carriages The carriage is the component which moves along the rail and normally runs on small diameter rollers. In most designs the carriage is pulled by a cable or chain, or driven along the inclined rail by a rack and pinion system or other drive arrangement. Most domestic carriages have a seat with arms and a footrest. Some special models have a stand-on platform also known as a "perch" seat. For users with shorter legs a short seat can be fitted, to make the lift more comfortable to sit on. Seats can be tailored to suit individual needs. The conventional layout for a typical domestic stair lift is to have the seat at right angles to the rail so the user travels "sidesaddle". At the top of the staircase the seat can be swiveled, commonly through around 45 degrees or 90 degrees, then locked in place to allow the user to alight from it onto a landing. Stair lifts are available with either a manual swivel or a powered swivel, depending on the users ability. Most swivel seats have a safety switch so the stair lift won't move unless the seat is locked into its travel position. Special models with seats facing the bottom of the staircase have been produced for users with spinal or other conditions which prevent use of the conventional seat layout. More room is need on the landing with these special seats. Popular types Straight-rail stair lifts These are the most common type of stair lifts used in private dwellings with straight stairs and have a straight rail (track) which is attached to the steps of the staircase. Straight-rail stair lifts