It is usually a swivel chair, with a set of wheels for mobility and adjustable height. Modern office chairs typically use a single, distinctive load bearing leg (often called a gas lift), which is positioned underneath the chair seat. Near the floor this leg spreads out into several smaller feet, which are often wheeled and called castors. Office chairs were developed around the mid-19th century as more workers spent their shifts sitting at a desk, leading to the adoption of several features not found on other chairs.