To return to the same area on the disc, each area is given a unique address consisting of three components: cylinder, head, and sector. Horizontal addressing is accomplished by assigning numbers to the concentric circles (cylinders) mapped out by the heads as the positioning arm is stepped radially across the surface, starting with 0 for the outermost circle. By specifying the cylinder number the controller specifies a horizontal or radial address component of the data area. Rotational addressing is accomplished once a head and cylinder have been addressed. The desired sector around the selected track of the selected surface is found by counting address marks from sector 0. Vertical addressing is accomplished on a disc, by assigning to the heads numbers 00 through XX in consecutive order. By specifying the head number, the controller specifies the vertical address component of the data area.