Abstract: A high thermal efficiency window which is comprised of a low conductivity ame carrying a spaced pair of panes or lites. The space between the panes is divided into a series of vertically spaced-apart cells by a blind structure disposed between the panes, the slats of the blinds being essentially in contact with the interior faces of the panes whereby convection losses are minimized. The device is characterized by the blind being shiftable between an open or see-through configuration and a privacy configuration, the longitudinal edges of the elements defining the blinds being in contact with the interior faces of the panes in both said see-through and privacy configurations thereof.