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Mullion

The member that separates two adjacent windows in the same opening.

A mullion is the vertical or horizontal member between two window units that sit side by side or stacked in one opening. It is part of the structure around the windows, not part of a sash.

A single window has no mullions. The bars dividing its glass are muntins.

Bay window in plan, seen from above, outside at top

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Bay window, in plan
Two double-hung windows side by side in one opening, seen from inside

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Two double-hung windows in one opening

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