I just recently learned to quilt and for the backing of the quilt, I used a king size sheet for my queen sized bed. I wanted a solid color and didn't want to spend $4 a yard when I could get a sheet at Walmart for $5 and get the color I wanted. I then tied it and put a binding on it and my quilt was done.
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I make many quilts for charities, as well as for other gifts. The perfect backing is a sheet. Twin size is the right size for the quilts our church makes for charities...