What does the abbreviation "d" in "16d" represent?

The abbreviation "d" in "16d" represents "penny." The term "penny" originates from the historical practice of measuring the length of nails based on how much one hundred nails of a certain size cost when they were first sold for the penny currency. For example, a "16d" nail is a nail that is 16 pennies worth, meaning it is approximately 3.5 inches in length. This system of measurement has been retained in many construction contexts to specify the size and type of nails, and it is widely recognized in the home improvement and contracting industries. Understanding this terminology is essential for contractors to select the appropriate fasteners for their projects.