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1. A unit denoting the length of a nail; for the common nail, and others which have been standardized, it also is an indication of the shank and head diameter.
2. (Abbr. d) A suffix indicating the size of a nail; the size specifies the length of the nail and the number of nails per pound, e.g., a 2d nail is 1 in. long and there are 875 per pound.