What is the primary use of a speed square?

The primary use of a speed square is to mark angles. This tool, often triangular in shape, features a 90-degree angle and a series of markings that allow carpenters to easily and accurately create various angles. It is particularly useful for framing and roofing applications, as it helps in laying out angles for rafters and other structural components.

While measuring lengths is a function of many tools, the speed square excels specifically in marking out angles for cuts and layouts, such as the common 45-degree angle used for trim work. Checking level typically requires a different tool, like a level, to ensure horizontal or vertical accuracy. Pounding nails is a task done with a hammer and is not related to the functions of a speed square. Therefore, the distinct role of marking angles highlights why this answer is the most appropriate choice.