Q: Why are nails designated in “penny” sizes?
A: “Penny” sizes originally referred to the price for a hundred nails in England in the 15th century; the larger the nail, the higher the cost per hundred. The terminology was adopted in the United States, written with a number and the abbreviation d for penny. 10d indicates a ten-penny nail.
Nails under 1-1/4 inch (brads) are sold mostly in small packages with length and wire gauge designations. Larger nails get the d or “penny” designation.
D is used to abbreviate “penny” because d is an abbreviation for denarius, a Roman coin similar to a penny that was originally used as the abbreviation in the United Kingdom.