backing
 

Can the seam on the backing run horizontally as opposed to vertically. I like my quilts to hang on the side and not come up under the pillows so the backing fits better this way.

If you are quilting it yourself, you can seam it anyway you want. Some long arm quilters require it one way or the other, so if you plan to send it out, you might want to consult the quilter first.

The seam can run any way you want. Longarmers prefer it to be horizontal, but we can usually mount the quilt sideways to make a vertical seam horizontal.

Any way you want it. My long armer doesn't care. I piece most of my backs so have lots of seams going both directions on the back.

It is recommended if sending out for longarm the seam be horizontal so the seam lays flat/ smooth along the bar-- if vertical it builds up bulk in the center with every turn loading it, causing floppy sides. If using a domestic machine,or hand quilting it doesn't matter so much

As stated above, it really doesn't matter if you are quilting it yourself. Check with your LAQ to see what she prefers but sometimes you have to use the backing fabric you have.