Selvage

The selvage (sometimes spelled selvedge) is the tightly woven, self-finished edge that runs along both long sides of a length of fabric, formed where the weft thread turns back on itself at the edge of the loom. It doesn't fray, which is exactly what makes it useful for more than just identifying which way is "up" on the bolt.

I use it constantly as a grain reference — the selvage always runs parallel to the lengthwise grain, so if a pattern piece's grainline arrow isn't lining up obviously, checking against the selvage settles it fast. I also keep an eye on the printed selvage text (fabric designer, colorway, sometimes a little printed color-swatch strip) before cutting it away, since it's occasionally useful for matching more of the same fabric later.

One habit worth having: always trim the selvage off before cutting pattern pieces from it, even though it looks stable enough to sew right into a seam. It's woven more tightly than the rest of the fabric, so it shrinks at a different rate and can pucker a seam after washing if left in.