Be sure to read what Marti Mitchell has to say. She explains several ways to join your sections together and some pros and cons of each. You can do the backing in sections,too. something I've done when I didn't want to do the backing in sections is to quilt my sections without a backing, or with a thin, lightweight muslin backing, sew them together to make the full sized quilt, and then added the backing in one piece. Then I quilted in the ditch vertically and horizontally to hold the backing. Consequently, you have more quilting on the front than shows on the back, but I have liked my finished product. Sometimes when I do this I use fleece for the back, and don't want to do a lot of quilting on it.