The "2B" designation on pencils refers to the two-part graphite grading scale that is used to measure the hardness of the graphite core. A 2B designation indicates a pencil is slightly softer and slightly darker than the scale's median grade.
The 20-step grading system, developed by the English pencil manufacturer Brookman in the 1900s, uses letters and numbers to measure the darkness of markings made with the pencil. According to this scale, which ranges from 9H (hardest) to 9B (softest), the hardest cores make the lightest markings, while the softest cores make the darkest. Most pencil manufacturers rely on internal scales to measure core quality and hardness and manufacture pencils to meet the demands of their particular market. Artists use this scale to choose the pencils that provide desired shading and darkness.