Which designation is used to indicate the sizes of nails?

The designation that indicates the sizes of nails is represented by the lowercase letter "d." This system is derived from the old term "penny," which refers to the price for a hundred nails in Britain. For example, a 10d nail indicates a size that traditionally cost ten pennies per hundred. This nomenclature helps standardize the sizes of nails, making it easier for builders and carpenters to identify and select the appropriate type for their projects.

Using the lowercase "d" is specific and recognized in the industry for representing nail sizes, thus providing clarity when communicating about nail specifications. Other designations, while they may relate to different aspects of tools or hardware, do not correspond to nail sizes in the same accepted fashion.