Warp, Weft & Bias
A woven fabric is created by weaving horizontal (latitudinal) threads under and over vertical (longitudinal or lengthwise) threads. The threads running horizontal are called WEFT threads and those running vertical are called WARP threads.
The lengthwise or longitudinal warp yarns are held stationary in tension on a frame or loom while the transverse weft (sometimes woof) is drawn through and inserted over-and-under the warp. A single thread of the weft crossing the warp is called a pick. Terms vary (for instance, in North America, the weft is sometimes referred to as the fill or the filling yarn). Each individual warp thread in a fabric is called a warp end or end. The weft threads wrap the end warp thread to weave back again in the other direction. The end turning point creates the selvage - the edge that runs the length of the fabric.
When you lay out a pattern you need to ensure the grain lines run parallel to the warp thread.
Fabric that is "cut on the bias" is cut on a 45 degree angle to the warp and weft threads.This allows the weave to "move". Some garments are cut on the bias which allows the fabric to follow the shape of the body more closely. How well a fabric "drapes" also depends upon the thickness of the warp and weft threads and how tightly these are woven together.
Bias tape (or bias binding) is a strip of bias fabric with the long raw edges folded toward the center of the strip. Bias tape is great for attaching to the finished edge of a curve in your garment (like an armhole or curved neckline).