This square is as straightforward as it gets, in a very good way.
It reads in millimetres on both edges of both sides of the blade. The outside edges read from 0 at the outside of the square, while the inside edges read from 0 at the reference edge.
It is made with an all-steel construction and is as square as we can measure.
Finally, the inside reference edge has a small circular relief, to make sure that the edge is correctly referenced and an uneven corner doesn't interfere with accurate measuring and marking.