Boxes, cabinets, walls and entire structures require right angles to align adjoining surfaces and distribute structural loads. The right angle, also called a 90-degree angle, appears all across the carpenter’s realm--at the intersection of wall studs and wall plates, posts and beams and floors and walls. Because of the right angle’s prominent position in the carpenter’s trade, a variety of tools is used to quickly and accurately measure 90-degree angles. Become familiar with the types of tools used to measure right angles and choose the right tool for your particular carpentry project.

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## Carpenter’s Square

This large, L shaped tool marks and measures right angles during the framing of walls, rafters and floors. Just like the letter L, the carpenter’s square has sides of varying lengths—24 by 18, 24 by 16 or 12 by 8 inches. The sides of the tool meet at a precise right angle, allowing carpenters to hold it to surfaces, such as walls, and gauge the angle at which they meet. A carpenter’s square features increments of measurement stamped upon one of its sides, such as inches or centimetres. Carpenter’s squares range in price in 2010 from under £6 to greater than £19; plastic versions are cheap, whereas stainless steel squares can be quite costly.

## Rafter Square

The rafter square is not just any metal triangle--it’s a right triangle. The perpendicular angles of a rafter square meet at a precise 90 degree angle and the remaining side forms 45 degree angle to the others. One of the rafter square’s edges has a lip—the lip easily catches on the edge of a board, allowing a carpenter to measure a right angle across the board’s face. Rafter squares are made of both metal and plastic and are available in 7, 10 and 12 inch sizes. The price of a rafter square in 2010 ranges from below £6 to over £13.

## Combination Square

The combination square, like the rafter square, measures and checks both right and 45 degree angles. The primary component of a combination square is a straight edged ruler. A precision machined measuring head attaches to a channel routed across the centre of the ruler’s length. The head slides up and down the channel and can be tightened by thumb screw to remain stationary. Once stationary, the head forms a right angle to the ruler on one of its sides and a 45 degree angle to the ruler on the other. Combination squares are useful in both rough and finish carpentry and cost in 2010 between £9 and £19 depending on quality.