If a 3 1/2 inch long nail is required for nailing wood framing members together, what size nail should be used?

The correct choice is based on understanding the nail sizing system, which is designated by "d" (penny) sizes. The size of a nail correlates with its length; thus, nails are classified into these sizes for convenience in construction tasks.

For framing wood members, common practice suggests that a 3 1/2 inch nail is most appropriately categorized as a 16d nail. This size is traditionally used for such applications because it provides the right combination of length and holding power necessary for securely fastening framing materials together.

In contrast, other sizes such as 12d (which is shorter at 3 inches), 20d (which is longer at 3 3/4 inches), and 10d (which is 3 inches) do not meet the required specifications for the task described. Therefore, 16d nails are recognized as the most suitable for using with 3 1/2-inch lengths in this context, as they are specifically designed for heavier construction work, ensuring durability and structural integrity in framing projects.