-penny
(-d)
   an ending added to a number to indicate the size of a nail, as in "sixpenny (6d) nail" or "tenpenny nail." It's not clear exactly how this terminology began, although the usual guess is that tenpenny nails originally cost ten pence per hundred. There is, very roughly, a linear relation between the size designations and length: an n-penny nail is roughly (1/2) + (1/4)n inches long. This makes the tenpenny nail about 3 inches long, the eightpenny about 2.5 inches, and so on.

Dictionary of units of measurement. 2015.

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