Abstract: A method of preventing light from passing through a gap between a window frame and a window treatment disposed in the window frame, the window frame having a window opening and vertical sides includes the steps of providing an L-shaped light-blocking device with a mounting face, a retaining face extending substantially perpendicular to the mounting face, and at least one frangible width adjustment notch disposed lengthwise along the retaining face, and attaching the mounting face to one of the vertical sides of the window frame to have the retaining face extend from the vertical side between the window treatment and the window opening and block at least some light passing between the window frame and the window treatment.

Abstract: A light-blocking valance for a window treatment is an L-shaped member having a mounting face and a retaining face, the retaining face extending substantially perpendicular from the mounting face. At least one frangible width adjustment notch is disposed lengthwise along the retaining face for adjusting the width of the valance by breaking it along the desired notch. The mounting face is attached to a side of a window opening such that the retaining face extends outwardly from the side of the window opening to block light and retain the window treatment. When installed in a window opening, the retaining face is configured to be disposed between the window treatment and the window to keep the treatment away from the wall while also closing the gaps between the window opening and the edges of the window treatment.