Sidelight windows might seem challenging to cover, but Wendy Lami - Material Things offers a variety of window treatments that come in narrow widths:
Duette® Honeycomb Shades, are offered in almost any shape and size. Plus, they have the added benefit of a cellular construction that creates an extra layer of insulation at the window.
Silhouette® Window Shadings look great on small windows. They have S-shaped vanes that appear to be floating between two sheers. The sheers draw sunlight into the room, while the vanes tilt to control light.
Luminette® Privacy Sheers are a vertical window treatment that looks great on sidelights, featuring a sheer fabric face to let light in and soft fabric vanes to keep light out.
Vignette Modern Roman Shades are another window treatment option that's available in narrow widths.
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