Aka. Speed Square
7 inches
Article-How to use: https://www.popularmechanics.com/home/how-to/g1209/how-to-use-a-rafter-square/
Video-How to use: https://www.youtube.com/watch?v=eNH_BhjwhV8
Types of Squares: https://www.doityourself.com/stry/infoaboutsquares