When I make half square triangles, I cut a 9 1/2" square from both fabrics I intend to use; put right sides together and mark a diagonal line from corner to corner and also from the opposite diagonal. I use that line to sew 1/4" seams on either sign of the diagonal lines. Then I cut the whole square in 4 quarters, then cut the lines left between the seams and end up with 8 3 1/2" half square triangles.