Fabric is woven with fibers, and the direction of the fibers is called the "grainline".
- The fibers that run parallel to the selvage are the "weft", or quilters call it the Length of Grain. These are the strongest fibers in the material, and they have little to no stretch or give.
- The fibers that run from selvage to selvage are the "warp", or Cross Grain. These are the weakest fibers in the material, and they have a small amount of stretch or give.
- A diagonal cut across the warp & the weft is called Bias, and it has the most stretch of all.
Why should you care?
1. Marti Michell is a self-described "grainline geek". Since the length of grain has little to no stretch, she cuts all of her pieces on length of grain so that her project is guaranteed to be nice and flat. If you cut your borders on the length of grain, you won't have to seam them for length. Con: Since there is no give, your piecing needs to be deadly accurate. There's no "easing" allowed. Using length of grain for piecing and borders also requires that you purchase more fabric.
2. Pieces cut cross grain have a little bit of stretch. If your piecing tends not to be 100% accurate, you can "ease" blocks to sashing and borders. You will require less fabric if you cut all your pieces cross grain, but any sash or border longer than 42" will have to be seamed.
3. Bias is extremely flexible, and is required if you have curves or other attributes that require the fabric to bend. Of course, the diagonal seam in a half square is on the bias. This is why you need to use caution when ironing half squares or any other piecing that has an angle in it (like flying geese). "Ironing" instead of "pressing" along a bias-pieced seam can cause distortion in the block. But cutting on the bias is very handy for applique or for binding a quilt with curved edges.
4. Tearing fabric is easier if you tear along the length of grain rather than the cross grain. Think about it; by tearing on the length of grain, you are breaking the weaker (warp) fibers. You have a minimum of damage or bruising. This is why I tear my selvages off when piecing my backing. If you tear across the grain (from selvage to selvage), you are attempting to break the strongest (weft) fibers, and the resulting damage (or "bruising") to the remaining fabric can equal a half inch or more. This is why we rotary cut our fabric in the store instead of tearing it. Some quilt shops still tear fabric from the bolt. If you ever encounter this, you can ask to have your fabric cut, and if they won't, I suggest that you purchase extra fabric to allow for the "bruising" that will result.
Even if you don't have the selvage on a piece of fabric anymore, you can use the stretch test to determine the grainline of a piece of fabric. Go ahead, give it a try!