A framing square consists of two legs: a longer leg, called the blade, and a shorter leg called the tongue. On some squares, there are settings rules that help establish the pitch of a roof and rafter placement.
To read a framing square, place the longer leg against the top of a joist, and read down the tongue to a mark you have determined will be roof support. You use the blade as the distance measurement and the tongue for depth measurement.
On some squares, there is a third tongue with which you can establish the pitch and angle of the roof.