What does d with a nail size mean?

In the context of nails, "d" stands for "penny," which is a historical term used to indicate the size of nails. The number preceding the "d" (e.g., 8d, 16d) refers to the penny size, where a higher number indicates a longer nail. For example, an 8d nail is typically 2.5 inches long, while a 16d nail is around 3.5 inches long. This system helps in selecting the appropriate nail size for various construction and woodworking projects.