I tie my quilts, so backing goes on frame first, then the batting and then the top...two quilts ago I had first two layers on it then layed the top on and yup, you guessed it...backing was too small. Lucky for me I took the top off and looked in the chest and found a top my mom had pieced and it was smaller so put that one on instead and kept on with my endeavor. Then I went out and purchased another backing, big enough this time and continued on with the big quilt! As I said, I was lucky. Pinning three layers onto the frame is not fun so I'm glad I could use that backing with the smaller top.