Penny (unit)

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 1¼ 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:^[1] the larger the nail, the higher the cost per long hundred.^[2][3][4][5] The system remained in use in England into the 20th century, but is obsolete there today.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.

References

1. ↑ D. Nutt (1890). Archaeological Review: Volume 4. p. 322. In this connexion it is interesting to reflect that the proverbial "tenpenny nail" was a nail sold at 10d. for the long hundred. 
2. ↑ "Penny" (subscription required). Oxford English Dictionary (2nd ed.). Retrieved 2010-05-30. Applied to nails, such adjectives denote the original price (in 15th c.) per hundred; as fivepenny nail, a nail which cost 5d. a hundred, tenpenny nail, a nail costing 10d. a hundred. (These names persisted after the prices fell, as they began to do in some places before 1500, and they were eventually used to designate sizes of nails.) 
3. ↑ H. Littlehales (1905). Medieval Rec. London City ChurchCited in the Oxford English Dictionary under "Penny" with a quote from 1426-1427. 
4. ↑ "Penny". sizes.com. Retrieved 2010-01-10. 
5. ↑ Norman Scott Brien Gras (1918). The Early English Customs System. Harvard University Press, Cambridge (MA). p. 701.  Cited at sizes.com with a quote from 1507.