The Historic One Lite (HOL) series is designed to be “invisible”, - when painted the same color as the existing sash. All existing sash should be operable, to permit removal and/or cleaning from the interior. There is no hardware on the exterior. Typically, one or two panels are removable from the interior, and are secured by perimeter clips.
The Allied One Lite (AOL) series is used where the top sash of a double-hung is not operable, but the bottom panel needs to be inside-removable. The upper panel is outside-removable, and “invisible clips” (flush) are used on the perimeter.
Frequently used for 18th Century homes where narrow sight-lines are important. This stacked unit has a fixed (but removable) upper panel, and a lower panel which is inside or outside-removable, and which can interchange with a screen.