Which tool would you use to check whether a corner or joint is at a true 90-degree angle?

A square (such as a carpenter's square, try square, or combination square) is used to check and mark right angles, confirming that a corner or joint is a true 90 degrees. A level checks whether a surface is horizontal (level) or vertical (plumb); a tape measure measures length; and a plumb bob uses gravity to establish a vertical line. Matching the measuring/marking tool to the task — a square for checking 90-degree angles — is standard Shop Information content. Accurate squaring is essential for joints, framing, and layout work.