The hardness of the core is often marked on the pencil -- look for a number (such as "2" "2-1/2" or "3"). The higher the number the harder the writing core and the lighter the mark left on the paper. As the pencil core becomes softer (through the use of lower proportions of clay) it leaves a darker mark as it deposits more graphitei material on the paper. Softer pencils will dull faster than harder leads and require more frequent sharpening.
