The "d" sizing for nails has to do with a bit of history. "D" stands for penny, harking back to Roman rule in England, when a denarius was also the name for an English penny. This was when nails were hand-forged, and the marking system indicated length of a nail. So, for instance, a 16d nail is 3.5 inches in length. Some say one 3.5 inch nail was worth 16 pennies, while others say that perhaps 100 3.5-inch nails were worth 16 pennies. Either way, this coding system still exists today, with both length and d-sizing on retail boxes.