A pencil’s location on the HB graphite grading scale depends on the hardness of its graphite core. The hardness of the graphite core is often marked on the pencil — look for a number (such as “2” “2-1/2” or “3”) — and 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 graphite material on the paper. Softer pencils will dull faster than harder leads and require more frequent sharpening.
You might see other markings on pencils. Most pencil manufacturers outside of the U.S. use the letter “H” to indicate a hard pencil. Likewise, a pencil maker might use the letter “B” to designate the blackness of the pencil’s mark, indicating a softer lead. The letter “F” is also used to indicate that the pencil sharpens to a fine point.