Find the two long edges
Selvages run along the sides of woven yardage. The fabric shop cuts across the width when measuring your purchase. A selvage may carry the fabric name, printed color dots, or small holes.
Measure after trimming
The tightly woven edges can behave differently from the fabric between them, so they are often removed for sewing and quilting.
For example, if fabric measures 44 inches across and you remove 1 inch from each side, you have 42 inches of usable width. The amount to trim varies; measure your own fabric before entering its usable width in a calculation.