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This is the stuff that is sandwiched between the pretty parts of your quilt. There may seem like a billion options to choose from (and there are), making the process fairly intimidating. But sticking to the basics now will ensure your success later. Mainly, you need to consider the loft and fiber of your fabric.  Loft is a fancy term for how thick your batting is. A low loft means the batting is thin. A low-loft fabric is easier to work with, but will result in a thinner product. Fiber is what material your batting is made of. Polyester, 100% cotton, and a cotton/poly blend are your three most common options and none is necessarily better than the others. Wool and silk are also available, but they're a bit pricier. And a recent mover on the scene is bamboo, but that's just strange.  Polyester - A cheap option that's better for hand-quilting if it's low-loft. It doesn't need to be quilted closely together, though it does tend to shift and the fibers can migrate to the edges of the quilt over time. Cotton - This is a good option for machine quilting. It should be quilted closely. It will shrink a bit, but it shouldn't pill. The 100% kind feels like flannel. Cotton blend (usually 80% cotton/20% polyester) - Probably the best option, if you had to pick. It's not super expensive and it doesn't shrink as much as the 100% kind. It's good on the machine, too. This should be the biggest part. The batting should be smaller than your quilt back and larger than your quilt top. The quilt top will be the smallest. Just so long as it’s a few inches bigger on all sides than your front, you’re fine.  The reason a back needs to be bigger is because you’re usually quilting from the top of the quilt and the batting and backing can shift slightly underneath. The extra inches are your insurance policy that your back doesn’t suddenly become smaller than the front. Basting a very important step in the quilting process. It seems tedious, but doing it carefully will result in a professional-looking finished project. Basting is a way to temporarily hold the three layers together while you quilt.  Iron the backing fabric and lay on the floor face down. Carefully pull the fabric taught (but don’t stretch it) and tape it to a hard, flat surface. Smooth the batting and lay your quilt top over the batting. Press both layers together to get all wrinkles out. Doing so also helps the quilt top adhere slightly to the batting. When top and batting are smooth and flat, carefully roll the two together. Bring the top and batting to the quilt back and carefully unroll on top of the backing, smoothing all wrinkles as you unroll.  Make sure you can see backing fabric around all four edges of the quilt top. Here's where you have some options. That is, if you're machine quilting. You can always baste in the traditional sense or use spray baste, too. Pin quilt top every few inches starting at the center. Use basting pins -- they're curved and easier to manipulate. When pins are in place, remove the tape and check the quilt back to make sure things are tight and flat. If there are puckers or excess fabric, now is the time to fix the problems. If the fabric is loose when you start quilting, there will be tucks or puckers in the quilting. There is no way to adjust the back once you start sewing without a lot of headaches or time with the seam ripper. (However, using a busy, patterned fabric for the back will help to hide any small mistakes.) There are many options for machine quilting.  The first is to let the seams/fabrics themselves be your guide. Sewing next to the seams themselves is called "quilting in the ditch." If you want to create more visual interest in the quilt you can stitch lines or patterns in other directions. It’s a good idea to start quilting from the center and work your way out. Because it’s hard for all that bulk to fit through your machine, roll the sides in. You can un-roll as you work toward the edges. You may want to use a walking foot when quilting.  It’s not necessary, but it helps feed the layers of fabric evenly through the machine.
Choose your batting. Cut out your backing. Assemble your layers. Keep them together. Start basting.