The number two pencil is the standard pencil used in countless schools by countless students over the years. But why is that? Why, when the school supply lists are sent out at the beginning of the year, is it that teachers specifically request that students have number two pencils? And why is there a number for a pencil anyway?
The answer has to do with the graphite in the pencil. (Another fun fact, pencils contain graphite, not lead, even though they are often referred to as lead pencils. This is because when the English found a large graphite deposit in England in the 16th century, they were unsure as to what it was, though they found it was useful for writing, and so they called it pencil lead.) The hardness of graphite varies and therefore a scale is used to describe the varying levels of hardness for graphite. The number two is a mid-range graphite hardness, which makes it suitable for academic use because it makes a mark that is sufficiently dark (and therefore able to be read by scantrons) and also easy enough to erase. The higher the number, the harder the graphite. Harder graphite makes a lighter mark and is also more difficult to erase.
Different scales of hardness developed in America and England, so the English (as well as the rest of the world) use a system of letters to grade their pencils, while the United States uses numbers. However, the United States does use letters to grade pencils that are made for artists, which is why you might see pencils marked B, HB, F, H, or even 2H. For such a simple instrument, things can get pretty confusing!