A carpenter's square that can have the arms thereof separated for ease of storage and joined together during use includes a over-center latch arrangement designed to be no thicker than the thickness dimension of the arm members and which, when operated, tightly clamps the arms of the square at right angles to one another. A first arm of the square has an opening formed through the thickness dimension thereof along with alignment bores extending inwardly from a side edge thereof on either side of the opening. The second arm includes the over-center latch pivotally mounted in a slot or channel and includes a latch plate pivotally joined to a coupling member having a head thereon designed to fit within the opening on the first arm member. When the latch plate is manually actuated so as to reside within the slot of the second arm, the head on the coupling member cooperates with an edge defined by the opening in the first arm to tightly clamp the first and second arms one to the other.