When to Use Bias
Bias runs diagonally between the lengthwise and crosswise grain lines of a woven fabric. The "true" bias runs exactly at a 45-degree angle to the grain lines and has the most stretch in a woven fabric.
Purely Visual Reasons
You can also cut directional fabrics, such as plaids or stripes, on the bias for purely visual reasons. Cutting these types of fabrics on the bias results in a distinctive look for your quilt; a bias binding cut from a striped fabric creates a "barber pole" effect.
Cutting Bias Strips
How do you cut bias strips? Begin with a fabric square or rectangle. Using an acrylic ruler and rotary cutter, make a cut at a 45-degree angle to the left edge. Measure the desired width from the cut edge, then cut parallel to the cut edge. Handle bias strips carefully to avoid stretching or distorting the fabric.