On the framing square blade face using the common rafter length per foot run line in the table look along the blade under the inch number 18 we see 21 63.
Roof rafter layout framing square.
18 12 roof slope rafter lengths let s figure the rafter length for an 18 in 12 roof.
I marked it in red to make it easier to spot.
The standard two foot framing square also referred to as a carpenter s square and historically as a steel square is an l shaped tool that is used to mark angles for cuts used in building framing particularly roof rafters stair stringers and many other cuts or angles other than 90.