A mechanical pencil is defined by Dictionary.com as "a pencil for holding lead that can be extended by mechanical means." Mechanical pencils use a thin, replaceable lead that's inserted into a tube in the pencil. Being able to extend the lead during use provides you with a sharp point to create precise lines when drawing. Unlike a regular pencil, a mechanical pencil never has to be sharpened.
Leads for mechanical pencils are available in different grades and sizes. A lead's grade indicates its hardness. A grade is determined by a number, a letter or a combination of both. The English grading system from darkest to lightest are the 9B, 8B, 7B, 6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H, 7H, 8H, 9H leads. A softer lead, such as the B leads which stand for blackness, produces a darker shade and a harder lead, such as the H leads which stand for hardness, produces a lighter shade. Lead sizes range from 0.13 mm to 2 mm.
An H pencil lead is equivalent to a U.S. No. 3 pencil. An H pencil is used to create lines that are thin and dark. When drafting, for example, the H lead in a mechanical pencil is used to draw dimension and extension lines, to trace images from a stencil or when using a stencil for lettering. The H pencil can also be used to create grid lines which can later be easily erased.
An F pencil lead, which stands for "firm" is equivalent to a U.S. No. 2.5 pencil. An F pencil is used to create thin lines that are slightly darker than an H pencil. When drafting, for example, the F lead in a mechanical pencil is used for drawing lines that need to stand out, such as walls and borders. Since an F pencil is slightly darker than an H pencil, it leaves more of a trace when erased.