Preparing a quilt backing

A number of quilters have asked me about my quilt backings.  They want to know why they are cut on the diagonal and then sewn back up again.

If a picture is worth more than a thousand words, how much is a whole bunch of photos worth?  Here is the method taken from a John Flynn tutorial that I use. 

The 42" width of fabric was just a bit too narrow to use as backing for this quilt:

So a length of fabric was cut on the diagonal:

Watch what happens when you slide the right hand side down.  The backing is now narrow and longer:

Now slide the right hand side up.  The backing is shorter and wider.  Which is what is required for the quilt top.

Is it wide enough? 

Well there would be just enough providing no mistakes are made loading the quilt top onto the frame.  So I add a 4" strip of fabric to the diagonal cuts when I sew the left and right sides together.  I'd rather have a bit extra then find out that I was off a half inch or so at the bottom of the quilt.

Now why not just add a straight piece of fabric to one of the sides of the fabric?  Why the diagonal cut?  The answer is because the seams will not "stack" on top of each other when the backing is rolled up.  Much easier to get a smooth backing.

You can see how the seams will lay as the backing is rolled up on the frame poles.  No build up.