These are very thin fabric with a lot of stretch.  At first, I thought maybe I read the description wrong..100% cotton.
I rechecked and yes, that is exactly what it says.  These are nice and cool and will be great for summer when you don't really need a heavy pair of underpants. The weave is what gives them a stretchy feel.  The leg openings are covered in the fabric, so no elastic against your skin.