Warp and Weft Definition
Warp and weft refer to the threads that make up a loomed or woven fabric. Warp threads are the threads that run along the length of the yardage and parallel to the selvage. Weft threads are the threads that run from selvage to selvage (side to side).
Any fabric created on a loom will have a warp and weft thread, including evenweave, which has an equal number of warp and weft threads per inch, plain weave with its characteristically tightly woven warp and weft, and many other fabrics suitable for embroidery.
Nonwoven fabrics such as felt, vinyl, suede, and leather do not have warp and weft threads.
Why Is This Important?
Most of the time, when you go to embroider on fabric, you won't have to think about warp and weft. They will rarely affect your work. But it's good to know how your fabric might move, and that's a direct result of warp and weft.
Some types of stitching, such as drawn thread or pulled thread embroidery, involve working with warp or weft threads specifically.
You'll also find that some embroidery fabrics have very different patterns of warp and weft, as they are woven specifically for a type of embroidery, such as huck toweling for Swedish huck embroidery.
It's also important to pay attention to this when working on counted-thread embroidery or any type of stitching that requires evenweave fabric. Some linen fabrics look similar to evenweave, and they may even work for your project, but not all linen is created equal.
For example, sometimes the weft threads are finer than the warp threads or they may all be different thicknesses. This results in stitching that isn't squared or even. You can see this in action at the end of the Kasuti embroidery overview.
Stretch is an especially important thing to understand since you'll want to avoid distortion when you transfer patterns and hoop your fabric.
When pulled warp-wise, most fabric types will have very little stretch but pulled weft-wise, there will be a small amount of stretch. The warp threads are typically stronger, as they have to run the entire length of a bolt of fabric.
Fabrics with warp and weft threads have the most stretch when pulled diagonally, or on the bias. In some sewing situations, this is helpful, but in embroidery, it can cause distortion. To avoid this, keep an even tension on the warp and weft threads as you are transferring your pattern or placing your fabric in a hoop.
To get the straightest cut of fabric, follow the warp and weft as you cut. With looser weave fabrics you can pull a thread out from the entire width of fabric or length you are cutting, then cut the gap left by the pulled thread. You'll also use this technique for making a self-fringing edge.
Other fabrics will naturally tear along the grain if you make a small snip in the edge and then swiftly rip the fabric.
Updated by Mollie Johanson