Opposing Waves Cathedral Window Block PDF Download Block 4 of 9 in the Cathedral Window Quilt-a-Long. Block size is 12-1/2” square unfinished. Use a 1/4” seam allowance unless otherwise indicated. Finished quilt will measure 36“ square. Use Legacy™ by Pellon® 100% Natural Cotton or Soy/Cotton Blend Batting for a beautiful finish. Cathedral window blocks are traditionally about folding and topstitching to create beautiful blocks with a lot of depth.