In order to find the fabric’s true bias you need to be familiar with some other terms. The selvage is the light green strip at the top of the cotton fabric pictured below. Often it is a different color and has the name of the manufacturer printed on it. When you tug on the selvage, it does not “give”. Never use the selvage in your patchwork.
The straight of grain runs parallel to the selvage. When you tug on the straight of grain, there is little “give”. The cross grain is always perpendicular to the selvage. When you tug on this, the fabric will “give” a little more. This is important to note. When sewing on borders you usually want the fabric to remain flat (little “give”).
The fabric bias runs at an angle to the selvage. True bias is at a 45 degree angle to the selvage. When you tug on the bias, the fabric stretches. Care must be taken when sewing on the bias as the fabric can stretch and cause inaccuracy in your patchwork.
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TRANSCRIPT: Fabric Bias – Encyclopedia
I purchased this piece of fabric at the fabric store. As you can see, I have a little over a yard. They’ve cut the fabric on each edge, and this part, this edge is called the selvage. Sometimes the selvage has a different color and sometimes you can get the dye colors in the selvage. The selvage is the strongest part of the fabric. When you pull it, it really doesn’t move very much. This is called straight of grain. Anything that is perpendicular to this, is called cross grain. The cross grain has a little bit of give in it; not too much though. A little bit more than the straight of grain. So straight of grain, cross grain.
Anything that’s not on the straight of grain or the cross grain is called the bias. True bias is at a forty- five degree angle. Now, so the bias is going to be at this forty- five degree angle. The bias has the most give, as you can see. So if you want to test out and see if you can find the selvage, which is the straight of grain, or the cross grain, one of the easiest ways to do it is just to tug.