It's actually 3 layers, I had a little anxiety that it wouldn't FIT under the machine's pressure foot for about an hour as I assembled the first two rows. But once I had them sewn together, like the thickest seams I had to do for the quilt, I knew it would work. I used a sharp brand new denim needle and it was fine. But it was 6 layers at it's thickest points. 2 denim and 4 flannel. So, the back side is identical to the front in that each block is front and back a puzzle block or a blue block. In the middle sandwiched between them is a layer of denim too. It's awesome! but extremely heavy, my next one will be too as I have already started the quilting process on the Leppard Quilt, got over half of them done with half the X design, and today I'll do the rest and finish the X design. I surely am taking my time and enjoying both projects, I just kinda worked fast.