“Another surprising development in the history of the pencil took place in the United States in the 1840s. While working at his father’s pencil factory, transcendentalist writer and philosopher Henry David Thoreau — best known as the author of Walden, a landmark series of essays on self-reliance — helped conceive of a new numbering system to describe the lightness and darkness, as well as the hardness and softness, of a pencil’s graphite. Labeled from numbers one to four, this new notation system took off nationwide, and the now-beloved No. 2 pencil became most popular due to the perfect balance of those qualities.”
I use #2 only.