Before Laying out the Pattern
Waistbands – cut on the lengthwise or crosswise grain?
When a fabric shrinks, often it shrinks more along one grain direction than another.
So if you cut the legs and waistband along the same grainline, when the fabric shrinks or tightens the shrinkage distance at the waistline will be different for the waist band and the pieces that join it.
Kathleen Fasanella posted an excellent explanation here with diagrams for pattern layout. It’s relevant to all waistbands, not just jeans or trousers.
For the home stitcher, a solution is to preshrink the fabric before cutting – wash and dry at least 3 times at the highest temperatures you will use for laundry, The down side of laundering before sewing is that the fabrics like denim will not be as crisp or easy to sew as they are before washing.
Twill Weave Fabrics (eg. Denim) – Avoid Fabric Skew and Leg Twist
Leg twist and skewing of fabric happens as a natural shifting as the fabric moves or shrinks after washing. The fabric skews in the direction of the weave which may be left or right handed.
Skew and Twisting leg was a common problem with RTW denim clothing and since the late 1970s manufacturers often compensate for the skewing effect during the weaving process. The fabric available to the home stitcher often doesn’t come with this information so what can be done to prevent legs from twisting?
In a factory setting the denim or twill weave fabric is tested for skew and performance by sewing pockets on tubes then washing them. The outcome of the tests affects how the pattern pieces are laid out on the fabric so that front and back pieces to be joined together work with the skew, much like an old dressmaking tip for laying out skirt pattern pieces to control the bias on sloping seams.
For the home stitcher, pre-shrinking by washing and drying 3 times will take care of most of the skewing caused by shrinkage but not that from the natural hang, so take care with the layout of the jeans leg and yoke pattern pieces. An example of the layout for counteracting leg skew is shown here.