Pencil Leads
Refill diameter has to match the barrel, and the figure is printed on the pencil as well as the tube. A lead sized for one barrel jams or rattles in another, and a drawer of mixed refills becomes useless. Once the size is right, grade follows the work: harder leads for technical lines, softer ones for shading, though very soft leads snap in fine diameters. See mechanical pencil leads, mechanical pencils and erasers.
