* Medieval England Origins: The most widely accepted theory traces the term back to medieval England. The "penny" (pence, abbreviated as "d" for denarius) denoted the price of 100 nails of a specific size. So, "sixpenny nails" would have cost six pence for 100 nails.
* Weight Connection: Initially, the penny designation related directly to the weight of 100 nails. For example, 100 sixpenny nails would weigh six pounds.
* Standardization Challenges: As time went on, this weight correlation became less consistent. Nail-making techniques evolved, and different manufacturers had variations in their nails. While the penny designation remained, it no longer perfectly matched the weight.
* Length Correlation: Eventually, the "penny" designation became more directly associated with the length of the nail, even though the original weight meaning was lost. This is the system we generally use today. For example, a 10d nail is longer than a 2d nail.
* Modern Use: The penny system is still used in the United States and Canada, but it can be confusing for those unfamiliar with it. A higher penny number indicates a longer nail.
In summary, construction nails are called penny nails because the name stuck over centuries from the time when the penny number indicated the price (and subsequently the weight) of 100 nails of that size in medieval England. Although the original meaning is lost, the term continues to be used to indicate nail length.