I've drafted a men's pattern and includes a curtain, what is a curtain related to men's trousers and not the one in connection with a window.

The "curtain" is a folded bit of fabric added as interfacing to the waist band.

It is typical of a tailored man's dress pant. It supports the waist.

Usuually made of a tightly woven cotton it can be seen inside any well made RTW dress pant.

Kaaren

Thanks for that Kaaren