Pull a piece of woven cloth along its selvedge and it feels almost like paper. Pull it across, thread against thread, and it gives a little. Pull it on the diagonal and it stretches a long way and then springs back. This is not because the yarn is elastic. It is because the cloth is a grid, and a grid can change its angles without any thread getting longer.
Take a square of woven fabric and pull two opposite corners apart. The square becomes a rhombus: the threads slide over one another at their crossings, the angle between warp and weft closes in one direction and opens in the other, and the whole piece grows along the direction of the pull. This is shear, and in a woven cloth it is the dominant way the material deforms. The stretch is real, but it is geometry rather than elasticity.
Shear is greatest at forty-five degrees to the threads, which is what tailors call the bias. Cut a panel so that its long direction lies on that diagonal and the panel will drape around a curve, cling where it is pushed, and fall back into place. Cut the same panel along the straight grain and it will hang like a board: fine for a structured jacket, hopeless for a dress that is meant to skim a hip.
The price of the give
Bias-cut cloth keeps stretching, and it does not fully recover. A panel cut on the diagonal and hung vertically will grow longer within hours, and it will keep growing for a day or more as the shear works its way down the piece. The growth is not uniform: the parts where the diagonal is steepest grow most, so a hem cut straight on the table becomes wavy on the hanger.
Every workshop that works on the bias deals with this the same way — by waiting. The cut pieces are hung, sometimes weighted, for a day before any fitting, and the hem is marked and cut only after they have stopped moving. A dress that is measured, cut and sewn in one sitting will be a different length by the time it is worn, and the difference is not small: several centimetres is ordinary in a full-length skirt.
Sewing it is awkward for the same reason. A bias seam stretches while the machine feeds it, so the two layers creep past each other and the seam comes out longer than planned and slightly curved. The remedy is to stabilise the seam before it is sewn — a strip of tape, a line of stay-stitching, or a sheet of tissue paper under the fabric that is torn away afterwards. Once the seam is closed the stretch is locked in, and the garment holds the shape it was sewn into.
What it costs in cloth
Bias cutting is expensive before a single stitch is made. Straight-grain pieces nest tightly, because their edges follow the same grid as the fabric and can be packed almost without waste. A bias panel lies at an angle to that grid, so its corners leave triangular offcuts that cannot be used for anything else. A bias-cut dress can easily take half again as much fabric as the same design cut straight.
There is a second cost that shows up later: a bias garment cannot be let out or taken in as freely. Altering it means cutting across the diagonal in a new place, which changes how the whole panel hangs. A straight-grain seam can be moved a centimetre without consequence; a bias seam cannot.
The technique is not new, but it becomes fashionable in waves, usually when the silhouette calls for cloth that moves. It is also why the dresses of a given decade look soft in photographs and stiff in others: the difference is often not the design but which way the pattern was laid on the cloth. A designer who wants a fluid line has to accept the extra metre of fabric and the day of hanging.
Compare it with the opposite approach, in which nothing is cut on the diagonal at all. A garment made of rectangles gets its fit from the width of the loom and from belts and ties, and it hangs straight because every panel is on the grain. A pleated skirt takes the third route: it keeps the grain and creates shape by folding, so the cloth holds volume without any of it being on the bias.
Bias binding is the small-scale version of the same idea, and it is everywhere. A neckline is a curve, and a strip cut on the straight grain cannot follow a curve without puckering. A strip cut on the bias will, because it can shear as it turns. That is why the tape sold in packets is cut at forty-five degrees: not for strength, but so it can bend in the plane of the cloth.
The useful way to think about it is that a woven garment has two available sources of stretch: the fibre, and the grid. Most everyday clothing uses the first and ignores the second. Bias cutting uses the second, which is free but not instant — you pay for it in cloth, in time, and in a hem that will not stay where you measured it.