Thanks! It was made from a bodice body of blue cotton, and the various diamond shaped scraps are appliqued on top. The diamonds of the body fabric are outlined in light blue stitching. The edging is bias tape which happened to be almost exactly the same shade as the body fabric.
Can you use a framing square to start a straight line across the width, using the length as one of the sides of the angle? Does that make sense at all? Then cut along the straight line with a long straight edge and a rotary cutter.