you should NEVER have just one seam running the length of your backing (top to bottom one seam) when this is loaded on the bars and rolled that center seam builds and builds extra layers and makes a large thick area, and the outside edges 'flop' making it difficult to quilt it without wrinkles. if you are only going to have one seam it should be horizontal so when loaded that seam runs along the bar evenly and does not build up all that extra bulk