Modern-day measurement originated in medieval times
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.