The straight Grain of the fabric is called the WARP, it runs up and down - the same direction as the selvedge (sides). If you were to pull (stretch) the straight grain of the fabric, you will see that it won't give. Now, if you were to pull on the fabric sideways it will stretch. This is called the WEFT of the fabric. left to right-WEFT. Have you ever noticed that when you are sewing cut squares of fabric that some sides will stretch and some won't? Thats because you are sewing the sides that have been cut on the weft side. side to side, left to right along the width of the fabric not the length. Hope this helps. I learned this in a textile class when I was in high school. Anne