Jeans are made from Cotton
We all know that. Normally this is correct. At least it should be.Unfortunately, more and more manufacturers (not reputable ones) resort to blends of all sorts.
Some use Polyester / Cotton blends, some use Denim stretch ( Helanca ) stretchable fibers.
Real jeans are made of 100 % Cotton. And we should all insist on buying the jeans.
More how they are manufactured : How are jeans made
