What is grain line and why is it important?
The grain of a fabric is the direction of the threads. Crosswise grain goes from selvage to selvage. Lengthwise grain goes parallel to the selvage. During the weaving of fabric, the lengthwise threads (along the selvage) are placed tightly in the loom to eliminate stretch. The crosswise threads (width of fabric) are not stretched when woven, leaving a little 'give' to the fabric.
Fabric piece cut along the grain line of fabricmake for more accurate piecing, less stretching and a stronger finished quilt.
Pieces cut diagonally across the grain are susceptible to stretching because there are no stabilizing threads along the edges. Bias tape is cut diagonally so that it will go around curves easily.