The number of window apertures is the total quantity of openings in the outer wall of a building where windows or similar light openings are placed. Individual apertures must be separated from each other by intervening structural elements (usually floorplates, columns, or wall sections) or by facade elements that are not themselves windows. Window framing elements do not constitute separating elements; in other words a single aperture may contain several window panes or window units.

When windows are arranged in continuous sheets or bands, a large group of windows can form a single aperture. If the windows are not interrupted by columns or stand in front of the columns, it may be possible for a single aperture to span an entire floor. However a continuous glass facade is not necessarily a single aperture, as sections of the facade covering structural members are generally rendered opaque and therefore do not constitute windows.

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.