Abstract: A square is disclosed which is especially adapted for the marking of angular dimensions on, or the transfer of angular dimensions from, rafters or other building materials and the like in that only one adjustment is needed to vary the angle being marked or measured. The disclosed square has a first elongate arm, a second elongate arm, and an elongate base having a distal and a proximal end, and a first elongate slot extending longitudinally intermediate its distal and proximal ends. The first elongate arm has a proximal end and a distal end which is pivotally attached to the proximal end of the base. The second has a proximal end, a distal end which is pivotally attached to the proximal end of the first elongate arm, and a second elongate slot extending intermediate its distal and proximal ends for intersection with the first slot.