This is a common technique in quilting to create corner triangles on a unit. Instead of cutting exact triangles for the the corners, squares are cut instead, which are then sewn to the corner along the diagonal. The seam allowance is then trimmed, and the resulting corner triangle is “flipped” up. This techniques has several advantages: First, it improves accuracy due to eliminating a bias edge on the corner triangle. Second, the math is very simplified verses the traditional technique, which requires use of the Pythagoream theorem. The disadvantage is that it wastes a bit of fabric in the process, unless you choose to save those cut corners and sew them into HST’s.