An elongate hollow rail for a fabric covering for an architectural opening, such as a bottom rail for a fabric roller blind or a top or bottom rail for a roman shade, having an upper portion with a lengthwise upper opening in the top of the rail and being characterized by a pair of lengthwise upper locking shoulders in the interior of the upper portion on walls on opposite widthwise sides of the upper opening, the upper locking shoulders being wedge-shaped and pointing inwardly of the upper portion and away from the upper opening, and the wall on a first widthwise side of the bottom rail extending above the wall on the opposite second widthwise side of the bottom rail about the openings and a method for attaching a fabric to a locking shoulder within an elongate hollow rail as described.