A chair is a stable, raised surface used to sit on, commonly for use by one person. Chairs often have the seat raised above floor level, supported by four legs. However, a chair can have three legs (in a triangle shape) or could have a different shape depending on the criteria of the chair specifications. A chair without a back or arm rests is a stool, or when raised up, a bar stool. A chair with arms is an armchair and with folding action and inclining footrest, a recliner. The design may be made of porous materials, or be drilled with holes for decoration; a low back or gaps can provide ventilation. The back may extend above the height of the occupant's head, which can optionally contain a headrest. The chair is of extreme antiquity and simplicity, although for many centuries and indeed for thousands of years it was an article of state and dignity rather than an article of ordinary use.