Many vintage jeans “suffer” from leg twist. This is simply a natural adjustment of the fabric, which tends to follow the direction of the weave.
Stefano Aldighieri, Director of Fabric & Finishing, at Levi Strauss & Co. explains it:
“Levi’s denim were mostly right hand twills; the twill line rises to the right. During the weaving process you basically ‘force’ the fabric to be straight, perpendicular to the selvage, but at the same time you give it this direction in the construction.
You lay and cut the fabric; in the early days LS&CO patterns were cut straight along the selvage. When you wash the garments, the fabric will try to follow the direction of the weave and will pull in that direction, hence the twisted legs, the result of the movement of the fabric. Because Lee started to use left hand weave denims, their legs would twist the other way.”
Leg twist was eliminated in the 1970s by skewing (which contorts denim to its after-wash shape) – and later revived with Levi’s Red and Engineered.
Source: Paul Trynka