Fabric in the U.S. is sold primarily in 3 widths: fabric made for quilting and apparel comes in 44″ and 58″. This is the width perpendicular to the selvage and parallel to the cut end. Fabric made for home decorating (curtains and upholstery) usually comes in 54″. These are all give or take an inch or fraction of an inch. But you rarely can ever use the full width of the fabric because of what the selvage looks like. Selvage means “self edge.” Self means of itself as opposed to something added to it. See these photos:
The pattern layout diagrams in commercial patterns show you how to lay things out (not always the best arrangement and you can adjust it). The pattern envelope tells you how much fabric you need in both the 44″ and the 58″ widths. If you make your own patterns, you can lay out your pattern on the cutting table/cutting mat/floor/whatever and figure out how much fabric that pattern will take. If you normally cut through two layers, that is, you have the fabric folded like it usually comes at the store with the selvages together, lay the pattern out how you would cut it with it going however long along the (imaginary) fold but only so wide. Since half of 44″ fabric is 22″, give yourself 20″. Since half of 58″ is 29″, give yourself 27″. This will compensate for any selvage weirdness. Sometimes the fabric near the selvage is distorted. Either it’s sort of bubbly or lumpy (won’t lie flat) or it’s tighter or the weave is skewed.
I hope this makes sense, so let me know if it doesn’t.