top wider than backing
 

I am making a quilt from sweaters. I put a 31/2 inch white border around it.In trying to put it together, I found out that the width is 2 inches wider than backing. How do I fix this?

Cut the backing in 2 and put a piece in that goes with it or creates contrast.

Thank you. That sounds like it could work. This board is always very helpful

A creative back always adds a new level of interest. Just add an interesting strip on the back. It can be anywhere across the back; not necessarily in the center. Have fun creating a second area of interest.

That happened to me. I put a boarder around the backing. It turned out beautifully!

I prefer less symmetrical solutions. I'd either seam a piece to the side or more likely, make a cut at 13.5" in & seam in a complementary fabric there following the Rule of Thirds. (31.5 + 2 + 8" extra for quilting/LA = 39.5. Then 39.5/3=13.67 which I round to 13.5 for easier measuring. I'd put a 10" piece in between the two pieces.
If it's a busy fabric or a large repeat, you can seam more of the same fabric at a repeat. Otherwise, just pick another fabric that goes.

Just make sure that your backing is several inches wider than your top and it will work fine.

This has another advantage - it is difficult to line up a top and backing 'exactly' - so if the contrast strip is deliberately on one side - it does not matter if it is a bit to the left or right of the 'target'

I piece most of my backs. Sometimes something as simple as an extra stripe or as complex as inserting lots of pieces leftover from the front. I think it keeps it interesting and makes basic width fabric big enough to use as a back!

I'd cut the backing into 2 parts but I like to cut off of the center to give a little character and have used pieces of fabric or blocks from the front to fill in. Someone's suggestion of a border around the backing is also a good idea.