The backing fabric provides structure and support for your quilt, so you want it to be wide enough to allow the quilt layers to lie flat without Pulling. Most standard quilting cottons are around 42-45″ wide, but extra-large quilts may require backing over 108″ wide. Otherwise, the pieces won’t lie smoothly and you’ll see puckers and folds in the backing. From my experience, using a narrow backing results in an unsatisfying finish that diminishes your beautiful quilt top design.