Frame consisting of an aluminum star-foot and plastic-covered gas spring mechanism. Available as fixed model with non-adjustable seat height or height-adjustable lift model. Chair sizes at a fixed height in accordance with din en 1729 or height adjustable. Seat shell made from double-walled, structured polypropylene (lupo) for comfortable seating with air cushion. With concealed seat fixing and grip hole. Equipment and options: glide elements or castors for hard and soft floors or 2c universal glide elements. Optionally available with foot ring or with particularly easy-to-use 3d rocker mechanism with soft or damped sideways tilt capability. Model 31505 with reduced weight gas spring mechanism suitable for children. With piggyback hook for chair suspension on tabletops. The following material groups are available to choose from: frame made of steel tube: m1, 2; seat and backrest: c1, 6, 7.