Almost every syllabus, teacher and standardized test points to the ubiquitous No. 2 pencil, but are there other choices out there? Of course!
Pencil makers manufacture No. 1, 2, 2½, 3, and 4 pencils—and sometimes other intermediate numbers. The higher the number, the harder the lead and lighter the markings. (No. 1 pencils produce darker markings, which are sometimes preferred by people working in publishing.)
The current style of production is profiled after pencils developed in 1794 by Nicolas-Jacques Conté. Before Conté, pencil hardness varied from location to location and maker to maker. Earliest pencils were made by filling a wood shaft with raw graphite, leading to the need for a trade-wide recognized method of production.
Conté’s method involved mixing powdered graphite with finely ground clay; that mixture was shaped into a long cylinder and then baked in an oven. The proportion of clay versus graphite added to a mixture determines the hardness of the lead. Although the method may be agreed upon, the way various companies categorize and label pencils isn't.
Today, many U.S. companies use a numbering system for general-purpose, writing pencils that specifies how hard the lead is. For graphic and artist pencils and for companies outside the U.S., systems get a little complicated, using a combination of numbers and letters known as the HB Graphite Scale.
"H" indicates hardness and "B" indicates blackness. Lowest on the scale is 9H, indicating a pencil with extremely hard lead that produces a light mark. On the opposite end of the scale, 9B represents a pencil with extremely soft lead that produces a dark mark. ("F" also indicates a pencil that sharpens to a fine point.) The middle of the scale shows the letters and numbers that correspond to everyday writing utensils: B = No. 1 pencils, HB = No. 2, F = No. 2½, H = No. 3, and 2H = No. 4.
So why are testing centers such sticklers about using only No. 2 pencils? They cooperate better with technology because the machines use the electrical conductivity of the lead to read the pencil marks. Early scanning-and-scoring machines couldn't detect marks made by harder pencils, so No. 3 and No. 4 pencils usually resulted in erroneous results. Softer pencils like No. 1s smudge, so they're just impractical to use. So No. 2 pencils became an industry standard.