A chair is an item of furniture typically constructed from wood, metal or, more recently, plastic which is designed to allow a human being to sit on it. A chair will almost always have four legs though three legged examples are known. A type of chair known as an armchair will not have legs but castors. A chair will always have an upright portion to rest the back against, if it doesn't have this it's a stool, rather than a chair. If it is long and can seat many people, it is called a bench. If a chair is solid, and/or joined to the floor, it is more likely to be called a seat rather than a chair, as in a bus. Such constructions are known from Neolithic times. Other chairs have the capability to sway gently front to back, and these chairs are commonly referred to as 'rocking chairs'.