number of window panes
The quantity of individual exterior-facing sections of glass in a building's perimeter walls.
The number of window panes in a structure or module is the total quantity of individual pieces of undivided glass in its outdoor-facing perimeter walls, excluding glass doors and skylights. Double panes (or triple, etc.) only count as single panes for purposes of this statistic.
The three window counts describe the same windows differently and rarely match. One opening in the wall is one aperture; the frame set into it is one unit; the sheets of glass in that frame are several panes. A traditional sash window is one aperture, one unit and a dozen panes.
A source that says a building has a number of windows has not said which of the three it counted, so a figure taken from prose belongs in whichever field its wording actually fits.
Insulated glazing does not multiply the pane count. A double-glazed sheet is one pane, because the field counts divisions of the window and not layers of glass.
Related definitions
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number of skylights
The number of distinct glass-filled roof openings admitting light, with connected panes forming a single skylight. -
number of window apertures
The quantity of distinct or continuous openings for windows in the facade or structural wall of a building. -
number of window units
The quantity of individually installed sections of windows with window frames in a building's perimeter walls.