In 1565 the German-Swiss naturalist Conrad Gesner first described a writing instrument in which graphite was inserted into a wooden holder. But the mass-produced pencils we know today wouldn’t have been possible without French inventor Nicolas-Jacques Conté. During the French Revolution graphite was in short supply, so Conté devised a mixture of clay and graphite, which he’d shape, then fire in a kiln, a method still used today.

The No. 2 pencil is ubiquitous, but have you ever wondered what the number means? The hardness of writing pencils, which is related to the proportion of clay (used as a binder) to graphite in the lead, is designated by numbers from one, the softest, to four, the hardest.

The standardized use of No. 2 pencils goes back to the 1930s, when IBM hired teacher and inventor Reynold B. Johnson to create a model of his prototype test scoring machine. The IBM 805, marketed from 1937 to 1963, graded answer sheets by detecting the electrical current flowing through graphite pencil marks. The use of No. 2 pencils for exams today is a holdover from that period—modern scanner technology can recognize marks made by pens and pencils alike.