Abstract: A rafter square is provided with a triangular base plate, having a flanged base edge portion, a side edge portion, and a hypotenuse edge portion. An extension arm is selectively movable from a retracted position, closely adjacent the hypotenuse edge, to an extended position that is in-line with, but extends beyond a distal end of the side edge. Various locking mechanisms are provided for selectively locking the extension arm in either or both of its retracted or extended positions.