The bias of the fabric is the diagonal 45-degree line between the horizontal and vertical grain lines. So, if you look at a piece of fabric flat in front of you and the selvage is laying right along the bottom, that would mean that your horizontal grain lines are also running perfectly straight across from left to right. This would also mean that your vertical lines are running perfectly up and down, creating a nice 90 degree angle to your selvage. The BIAS LINE would run at an exact 45 degree angle from the selvage.