12d

an abbreviation for "twelve-penny" that indicates a nail that is three and one-quarter (3.25) inches (8.3 centimeters) long. "Twelve-penny" applies to common, box, casing, and finishing nails. The term originates from the price per hundred nails; 12 pennies for 100 three-and-one-quarter-inch nails.

Alternate spellings and misspellings: twelve penny nail, 12d nail, 12-d nail, twelve-penny-nail