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; a wheelchair has wheels fixed to an axis under the seat.
Chair design considers intended usage, ergonomics (how comfortable it is for the occupant), as well as non-ergonomic functional requirements such as size, stacking ability, folding ability, weight, durability, stain resistance, and artistic design. Intended usage determines the desired seating position. "Task chairs", or any chair intended for people to work at a desk or table, including dining chairs, can only recline very slightly; otherwise the occupant is too far away from the desk or table. Dental chairs are necessarily reclined. Easy chairs for watching television or movies are somewhere in between depending on the height of the screen.

Ergonomic design distributes the weight of the occupant to various parts of the body. A seat that is higher results in dangling feet and increased pressure on the underside of the knees ("popliteal fold"). It may also result in no weight on the feet which means more weight elsewhere. A lower seat may shift too much weight to the "seat bones" ("ischial tuberosities").

A reclining seat and back will shift weight to the occupant's back. This may be more comfortable for some in reducing weight on the seat area, but may be problematic for others who have bad backs. In general, if the occupant is supposed to sit for a long time, weight needs to be taken off the seat area and thus "easy" chairs intended for long periods of sitting are generally at least slightly reclined. However, reclining may not be suitable for chairs intended for work or eating at table.

The back of the chair will support some of the weight of the occupant, reducing the weight on other parts of the body. In general, backrests come in three heights: Lower back backrests support only the lumbar region. Shoulder height backrests support the entire back and shoulders. Headrests support the head as well and are important in vehicles for preventing "whiplash" neck injuries in rear-end collisions where the head is jerked back suddenly. Reclining chairs typically have at least shoulder-height backrests to shift weight to the shoulders instead of just the lower back.

Some chairs have foot rests. Around 15% of women and 2% of men need foot rests, even at the 16-inch (41 cm) chair height. A stool or other simple chair may have a simple straight or curved bar near the bottom for the sitter to place their feet on.


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