One of the basic pieces of furniture, a chair is a type of seat. Its primary features are two pieces of a durable material, attached as back and seat to one another at a 90°-or-slightly-greater angle, with usually the four corners of the horizontal seat attached in turn to four legs—or other parts of the seat's underside attached to three legs or to a shaft about which a four-arm turnstile on rollers can turn—strong enough to support the weight of a person who sits on the seat (usually wide and broad enough to hold the lower body from the buttocks almost to the knees) and leans against the vertical back (usually high and wide enough to support the back to the shoulder blades). The legs are typically high enough for the seated person's thighs and knees to form a 90°-or-lesser angle. Used in a number of rooms in homes (e.g. in living rooms, dining rooms, and dens), in schools and offices (with desks), and in various other workplaces, chairs may be made of wood, metal, or synthetic materials, and either the seat alone or the entire chair may be padded or upholstered in various colors and fabrics. Chairs vary in design. An armchair has armrests fixed to the seat; a recliner is upholstered and under its seat is a mechanism that allows one to lower the chair's back and raise into place a fold-out footrest; a rocking chair has legs fixed to two long curved slats; and a wheelchair has wheels fixed to an axis under the seat.
An office chair, or desk chair, is a type of chair that is designed for use at a desk in an office. 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 casters. 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. There are multiple kinds of office chairs designed to suit different needs. The most basic is the task chair, which typically does not offer lumbar support or a headrest. These chairs generally cannot be sat in for more than a couple hours at a time without becoming uncomfortable, though they often offer more room to move than higher-end chairs. Mid-back chairs offer fuller back support, and with the right ergonomic design, can be sat in for four hours at a time or longer. Egnormic chairs in this category, such as the Herman Miller Aeron and the Steelcase Leap are comfortable for long periods. Some mid-back chairs in particular offer customization options that can allow for a headrest to be added. Executive or full-back chairs offer full back and head support. Many executive chairs are designed to be sat in for eight or more hours at a time. These are typically the most expensive office chairs.