The first pencil made with graphite within a wooden shaft was developed sometime around 1560. Generations of students have been reminded to bring their No. 2 pencils when taking a standardized test. Pencil leads are graded based on hardness, with a No. 2 pencil being softer than a No. 3, but harder than a No. 1. Pencil leads do not contain any actual lead, but are a mix of graphite and a filler, usually clay.
The first pencils were actually just chunks of graphite that were held in the hand and used to make marks on writing material. As pure graphite is very soft, the writing was very apt to smudge. In 1795 Nicolas-Jacques Conté mixed particles of clay with graphite to produce the first pencil leads. Some manufacturers use fillers other than clay in their leads, but the basic principle of a graphite writing instrument remains the same.
The hardness of a pencil is determined by the ratio of filler to graphite. The more filler, the harder the pencil and the lighter the mark it makes on the page. The lower the number, the more graphite the lead contains and the softer the pencil lead. Softer pencils leave more graphite on the page. In the United States, pencils are rated from 1 through 4, with the No. 2 pencil being the most commonly distributed. A No. 2.5 pencil is a little harder than a No. 2 and has a slightly lighter mark.
European pencils use a different grading system, combing letters and numbers. "H" means hard; "B" means black, and "F" means the pencil sharpens to a fine point. Numbers are added to the letters to indicate degree, so a pencil graded as "9H" would be harder than one marked as "8H." The European pencil grade of "HB" is equivalent to the U.S. No. 2 pencil.
Grading is done by each manufacturer, so there is no central authority overseeing pencil grades and setting uniform standards. While one manufacturer's No. 2 is harder than that same manufacturer's No. 1 and softer than that manufacturer's No. 3, that same statement cannot positively be made for pencils from different manufacturers. As a result, although theoretically a No. 2.5 pencil is harder, with a lighter mark than a No. 2 pencil, that may not be the case if they are produced by different manufacturers.