What is the purpose of a carpenter's square?

The purpose of a carpenter's square, also known as a framing square, is primarily to establish right angles. This tool is crucial in carpentry and construction, as it allows for the accurate alignment and layout of materials to ensure that structures are built squarely and efficiently. By providing a consistent method to check and create 90-degree angles, the carpenter's square helps ensure precision in projects such as framing walls or laying out designs.

The other options do not accurately describe the primary function of the carpenter's square. For instance, flexible lengths require tools such as measuring tapes or flexible rulers, and marking circular shapes involves tools like compasses or saws designed for that purpose. Additionally, assessing horizontal levels is better suited to a level tool, which is specifically designed to check level surfaces. Thus, while these other tasks are important in construction and woodworking, they do not align with the main function of a carpenter's square.