A framing square, also known as a steel square or a carpenter's square, is a common tool used by carpenters, general contractors, masonry contractors, and do-it-yourselfers. Actually shaped like an L instead of a square, a framing square is usually made of aluminum or other metal. It has a long arm that forms a right angle with a shorter arm. The longer arm is typically used to measure right angles, as well as provide on its surface tables and scales commonly used by carpenters in a wide range of projects. Anyone involved in a building project or a project that requires ready access to tables and scales will find a framing square tremendously helpful in many aspects of calculating angles, cuts, and pitches.

The framing square is so-called by many because it is an essential tool in all framing projects, including stair, roof, and picture framing. The narrower arm, called the tongue, is typically 1.5 inches (3.81 cm) wide. The wider arm is referred to as the blade and is usually 2 inches (5.08 cm) wide. Other than being used to assess the accuracy of right angles, it is also used to properly lay out and space rafters, stairs, and hip rafters.

A standard framing square typically measures 24 inches (60.96 cm) long by 16 inches (40.64 cm) long. There are variations, however, and both longer and shorter tools are available. Although the tool is believed to have been around for centuries, the first reference to its size was in the 19th century when it was described as 18 inches (45.73 cm) by 12 inches (30.48 cm).

Framing squares usually have three scales on them: diagonal, board foot, and octagonal. More modern framing squares also have degree conversion charts for a variety of pitches and their fractional counterparts. There is also usually a table of numbers called the rafter table on the face of the square. This enables a carpenter to quickly calculate cutting angles and construction factors based on the Pythagorean theorem.

Long used by European craftsmen and builders, the framing square received a patent in the United States in 1819. The man who patented it was a South Shaftsbury, Vermont, blacksmith named Silas Hawes. Rumor has it a peddler paid him for shoeing his horse by giving him some old saw blades, which he welded together to form the first carpenter's framing square in the country. He founded the Eagle Square Manufacturing Company in 1820 with associate Stephen Whipple.