One portion is 13 1/2" x 18 1/2" with a few inches portion of that cut and then the second part sewn to it is a triangle with length at the longest section 15" and tapering off to smaller, as I said, triangle..
You can see in the photo, where the little cut piece is on the bulk of the rectangular piece and then the triangle begins...and the seam is visible too...hope that helps..