In the United States, the length of a nail is designated by its penny size, written with a number and the abbreviation d for penny; for example, 10d for a ten-penny nail. A larger number indicates a longer nail, shown in the table below. Nails under 1ΒΌ inch, often called brads, are sold mostly in small packages with only a length designation or with length and wire gauge designations; for example, 1" 18 ga or 3/4" 16 ga.
Penny sizes originally referred to the price for a long hundred nails in England in the 13th century: the larger the nail, the higher the cost per long hundred. The system remained in use in England into the 20th century, but is obsolete there today. The d is an abbreviation for denarius, a Roman coin similar to a penny; this was the abbreviation for a penny in the UK before decimalisation.