Carpenter's use many tools to lay out a "square" or 90 degree right-angle. The steel square and combination square are the most commonly used squares for construction and structural designs.
The steel square is a tool that carpenters use. The "title" steel square refers to a specific long-armed square that has additional uses for measurement, especially of angles, as well as simple right-angles. Today the steel square is more commonly referred to as the framing square. It consists of a long arm and a shorter one, which meet at an angle of 90 degrees (a right angle). It can also be made of metals like aluminum, which is light and resistant to rust.
The wider arm, two inches wide, is called the blade; the narrower arm, one and a half inches wide, the tongue. The square has many uses, including laying out common rafters, hip rafters and stairs. It has a diagonal scale, board foot scale and an octagonal scale. On the newer framing squares there are degree conversions for different pitches and fractional equivalents.
Carpenter's squares are very much like steel squares.
A combination square is a tool used for multiple purposes in woodworking, stone-masonry and metalworking. It is composed of a ruled blade and one or more interchangeable heads that may be affixed to it. The most common head is the standard or square head which is used to lay out 90°(right) angles, 30° angles and 45° angles.
thumb|300px|right|How to Use a Carpenter Square