Question: what are 16 penny nails?
Answer: The “d” is associated with a Roman coin called the denarius, which was also the name for an English penny. ... Some say a hundred 3-1/2 inch nails would have cost 16 pennies and thus became known as 16-penny nails. Others say that the number and the “d” indicate that one 16d (3-1/2 inch) hand-forged nail cost 16 pennies.