Bias

Every woven fabric has two straight grains — lengthwise and crosswise — running at 90 degrees to each other, with essentially no give in either direction. The true bias runs exactly 45 degrees between those two, and it's the one direction across the weave that has real stretch and drape, because you're pulling threads diagonally against their own crossing points rather than pulling them straight.

Garments cut on the bias skim and cling in a way straight-grain cuts just don't — think 1930s slip dresses. But it's a genuinely harder technique than it looks: bias-cut fabric stretches unevenly as you handle it, hems can droop and grow longer over the days or weeks after cutting as the fabric relaxes, and seams need to be cut and sewn with real care to avoid rippling. I always let a bias-cut piece hang overnight before hemming it, for exactly that reason — cut the hem too soon and it'll be uneven again within a week.

Even short of cutting a whole garment on the bias, bias-cut strips are how you make binding that can curve smoothly around a neckline or armhole without buckling — a straight-grain strip just won't ease around a curve the same way.