Sewing Terms Selvage

How do you figure out the grain of a fabric for sewing? Why do you need to know?

I am an amateur sewer, and I often find sewing projects asking me to find out the grain of the fabric. I haven’t a clue how to do this, and haven’t been able to find any literature that gives an easy-to-understand answer. They mention selvage and that it’s parallel to that, but what does that mean??! Could anyone explain this to me in very, very simple terms. It would also be helpful to know WHY this matters when sewing something.

Here is a direct quote from my quilting book.
“When fabric is produced threads are woven in two directions, creating a length and a width. This is called the straight of grain. If you cut diagonally ac cross the grain (in triangle pieces) you are working on the bias. Bias edges must be sewn and pressed carefully, since they stretch easily. The long finished edges of the fabric are called the selvages. Always trim off the selvage edges since they can cause distortion of the block and are difficult to hand quilt through.”

Depending on what you are sewing the block comment probably won’t apply. However, the selvages are plain white or the same color of the base color of the fabric, and usually have printing on them. If you are sewing clothing this wouldn’t be attractive in use. The length of the grain goes along the selvage, while the cross grain goes from selvage to selvage. The bias of the grain would go from opposite corners (IE top left to bottom right) of the fabric.
Why it matters is the strength of the grain. Your length grain is going to be stronger and stretch less then your cross grain. The bias is the weakest grain since it cuts across both grains.

Good luck and I hope you enjoy your sewing!