A (very) Quick Lesson on Bias

If you are partial to A-line or circle skirts you've probably noticed that, over time, the sides of the skirt get longer. Why?

Fabric has dramatically different properties based on the way it's hanging. Imagine one piece of fabric, 20 yards long, on a loom. (Looms aren't 20 yards long of course, but this is just a visual.)

The threads that make up the fabric go in two directions. Some are 20 yards long and go the entire length of the fabric. Those are warp threads.

The others are woven in at right angles to the warp. They are only 20 - 60 inches wide or so depending on how wide the loom is. These are the weft threads.

Warp threads are stronger and more stable (less elastic) than weft threads. When garments are made, they are generally made so that the warp threads run vertically up and down the body and weft threads, which have more give, run around the body horizontally. (This helps accommodate for that last, wholly unwarranted, trip to the buffet.)

Now, sometimes a garment is cut "on the bias." When you are watching an awards show and the stars parade up the red carpet in those drapey, flowing, form-skimming silk gowns - those are likely bias gowns. Often times they are the dresses that look like "a very simple dress." Not so - bias gowns are the most challenging to make.Why? Because when garment pieces are cut "on the bias," they have little or no stability. Bias cut fabric sections have warp and weft threads that run diagonally on the fabric. There are, then, no threads that run directly from shoulder to hem, or side to side, and stability is lost. There's no anchoring.Bias fabric stretches and slips and slides and really "morphs" every time you touch it. I tell students that sewing bias silk is like sewing water.

So...what does any of this have to do with your skewed rayon skirt? Well, the skirt is cut with those stable warp threads running down the center front. But just due to the shape of the skirt pieces being triangular, the fabric on the sides has bias qualities. Only the middle section has threads that run from waist to hem. The flared edges do not.

So, over time, it stretches out on the sides, where there are no threads anchored to the waist. What's worse, if there is no stretch in the seam itself, then the fabric near the seam stretches and the seam doesn't, leaving a ratty looking puckery line along the seamline stitching. (I can't draw this..but you'll know it when you see it.)

Note how, in the gored skirt, left, the lengthwise fabric threads can be re-distributed over the smaller gore sections. Because the skirt is cut in more sections, you can place more threads that go vertically from the waist to the hem. You achieve the same fullness, but with less bias instability. And the small amount of bias instability that is present, is distributed evenly among 6 seams (in a 6-gore skirt) rather than two, as in the skirt drawing above.

When you are making a non-gored skirt yourself, you can compensate for future stretching by letting the garment panels hang and stretch before you sew them together. It would also be wise to sew any bias seam with some stretch built it...a narrow zigzag stitch or a serged seam to avoid that ugly puckering at the stitching line if stretching occurs.

For those with RTW that has stretched, it can be re-hemmed.