Chair

A chair is a portion of furniture for sitting, consisting of a seat, a back, and sometimes arm rests, commonly for use by one person. Chairs also often have legs to support the seat raised above the floor. Without back and arm rests it is called a stool. A chair for more than one person is a couch, sofa, settee, loveseat or bench. A take apart footrest for a chair is known as an ottoman, hassock or poof. A chair mounted in a vehicle or in a theatre is simply called a seat. Chairs as furniture are usually not attached to the floor and so can be moved.
The back often does not extend all the way to the seat to allow for ventilation. Likewise, the back and sometimes the seat are made of porous materials or have holes drilled in them for ornamentation and ventilation.

The back may expand above the height of the head. There may be separate headrests. Headrests for seats in vehicles are important for preventing whiplash injuries to the neck when the vehicle is concerned in a rear-end collision.


Design and ergonomics

This unusual rocking chair is made of rough wood to give it an old-fashioned look. Chair design considers intended usage, ergonomics, as well as non-ergonomic functional requirements such as size, stack ability, fold 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 essentially reclined. Easy chairs for watching television or movies are anywhere in between depending on the height of the screen.

Ergonomic designs distribute the weight of the inhabitant to various parts of the body. A seat that is higher results in hanging feet and increased pressure on the underside of the knees. 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".