In garment sewing, knowing how to place the pattern on the grainline of the fabric is key for a fit that behaves. But what is grainline, and why do we need to know about it for quilting? For the same reasons: how we cut our fabrics for our strips, appliques, and blocks makes a difference in the final quilt. The three ways to talk about grain are
warp or lengthwise grain, the threads which run parallel to the selvedge
weft or crosswise grain, the threads which run perpendicular to the selvedge. When you cut “width of fabric”, this is cutting along the crosswise grain.
and bias, which is at an angle between the warp and weft.
To see how this affects your quilt, think about how the fabric stretches on the different grainlines. Take a piece of fabric, and pull it gently between your hands along the lengthwise grain, parallel to the selvedge. Then readjust your hands so that you are pulling across the crosswise grain, and finally, across the bias. You should notice that there is very little stretch along the lengthwise grain, slightly more cross grain and much more strength along the bias.
For clothing and quilting pieces that need stretch to work properly (think a clingy skirt or bias tape applique, where the fabric needs to stretch to go around curves), knowing grainline is critical. Likewise, if you’re trying to get things not to stretch, like borders, it is often best to cut your strips along the lengthwise grain. But why is one way stretchier than another? This awesome “vintage” video with Georgia Bonesteel shows how cotton moves from the field to the final printing process. It’s a long video, but the part on how thread is turned into fabric begins at 9 minutes in, and talks about “twist”:
In an nutshell: the threads that are used as the warp have been given extra twist to make them stronger to stand up to the manufacturing process. This means there is also less “give” and makes them stretch less. This isn’t as important for the weft, so they don’t receive that extra twist which makes them more stretchy.
Hope this helps as you make decisions on which way to cut your fabrics!