it is a 3d-coordinate system made out of a vertical coordinate "head", a horizontal (or radial) coordinate "cylinder", and an angular coordinate "sector". head selects a circular surface: a platter in the disk (and one of its two sides). cylinder is a cylindrical intersection through the stack of platters in a disk, centered around the disk's spindle. combined together, cylinder and head intersect to a circular line, or more precisely: a circular strip of physical data blocks called "track". sector finally selects which data block in this track is to be addressed, and can be viewed as a sort of angular component.