The term penny nail originated in medieval times and is believed to describe the number of English pennies required to purchase 100 nails.
In the 15th century, 100 six-penny nails cost sixpence, 100 eight-penny nails cost eightpence, and so on, so that the larger the number, the longer the nail. Today, the term is used as a measurement of the length of a nail sold for wood applications.
Nails are designated by their penny size, using a number and the letter d (6d, 8d, etc.). The letter d is an abbreviation for denarius, a Roman coin similar to a penny.