The pillar dial is an altitude dial, sometimes called a 'column' or 'cylinder' dial; the old name was 'chilindre'. It consists of a vertical pillar, usually of constant cross-section, mounted on a base or suspended from a shackle. The surface of the pillar is divided by vertical lines, usually six, into compartments, each for use in a given pair of months, which are indicated at the top or foot of each compartment. The cylinder is surmounted by a cap inside which is pivoted a gnomon in the form of a blade; for storage this fits in a hole in the pillar. To use the dial, the cap is removed, the gnomon swung out horizontally, and the cap with gnomon replaced on the pillar. The dial is then held with the gnomon pointed towards the sun and the pillar turned until the shadow falls straight down the side of the pillar. The time is then read from the shadow of the gnomon's tip on the hour lines, which twist helically round the pillar. Pillar dials showing unequal hours are shown in manuscript drawings before 1350; the earliest surviving example, showing equal hours, is dated 1455.