When you are creating a board, you may find it difficult to get a perfect square unless you measure the wood using a double square. These devices are designed to be used to ensure that the squared angles of the board match exactly all the way around. This can be done using a number of different methods.
Step 1 - Set the Tool
The double square consists of a straight ruler and a sliding bar, set at right angles to each other. You may also have other features, such as a bubble spirit level. Take the tool, and slide the blade across until it is pointing at "0". You should then lay the ruler along the top of the tool, measuring the length.
Step 2 - Checking the Squareness
Slide the blade until it comes into contact with the side of the board. You should be able to read off the length of the board across the rule. You can then place the double square on the other side of the board, and the sliding blade should touch the board at exactly the same point. If this is not the case, then the board is not square.