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. Number 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.
Conté’s method involved mixing powdered graphite with finely ground
clay, 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 is usually the same, the
way 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 use a combination of numbers and letters known as the
HB Graphite Scale.
Testing centers prefer Number 2 pencils, because their 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 and softer pencils like No. 1 smudge. Because of
this and general wide acceptance, No. 2 pencils became the industry