Why are collar backs and wigan cut on the bias? The following exercise will explain: Fold a piece of woven fabric on grain. How many of the threads are folded in half? Half of them, right? Fold the piece on the bias. Now, how many of the threads are folded in half? All of them, right? Bias cut fabric is twice as stiff but weighs the same. It also spreads out, so is perfect for sleeve facings.