Abstract: The Mad Square invention has a detachable spring loaded marker device which accepts replaceable lead points and for which the pencil lead can be adjusted to the right length by tightening the adjusting screw. The detachable marking device can be slid along either of the two sides of the horizontal straight edge with ruler marks on each edge of the blade segment to conveniently draw the desired angle. The combination square has a rotatable straight edge which pivots from the corner joint to a 90.degree. and 45.degree. angle on each side of the horizontal position. Other angles may be selected and used with this device which has a protractor device to measure the precise angle of the blade. The straight edge can be rotated and locked in position to establish a precise pre-determined angle or can be used in determining the relative angle between two pre-determined points defined by the extensions of the pivotally attached blade and head of the carpentry square.