RICH555:
See if I can remember this correctly, It has been a long time since I heard the explanation. It originated in the older days. It was based on a specific numbers of nails, I don't remember the original amount. But let assume 10 nails. 10 nails of the smaller size, didn't weight very much. Therefore they cost 1 penny(symbolized as D) while 10 of bigger nails(say 4") weighed more, therefore cost more, 16 pennies(D) and so on. Thereby we have the designation of 2D, 4D, 8D, 16D nails. You will see on home improvement show, they refer to nail as 8D, but the correct usage, is to call them penny. When asking for nails at a hardware store, You ask the clerk for a pound of 8 penny nails but the box will be marked 8D. HTH