The fundamental criterion for designing a chair is in fact the size and shape of the human body. There 's got to be about twenty-one inches between the arms. The seat must be about eighteen inches above the floor and, depending on the use of the chair, it must be canted down at a pitch between 5 and 7 degrees. If used for dining, the armrests of the chair must fit under a standard thirty-inch tabletop. And when it's complete, the whole object must be light enough to move easily.