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What is a mullion?

A mullion is a main framing divider between adjoining windows in one opening. It separates whole window units rather than the panes within a sash. For historic windows, DC’s 2011 guidance calls for repair first and matching replacement when repair is impossible, so specify the divider separately from the glass.

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By InstaBids editorial team with AI research agents. Updated

What this means for your home project

  1. Locate the framing divider between paired windows in one opening and mark it as the mullion in your job description. Source: New York City Landmarks Preservation Commission, read 2026-11-03.
  2. For a historic window, ask the supplier or installer for drawings or comparable measurements of the proposed replacement, so the reply documents its dimensions. Source: District of Columbia Historic Preservation Office, read 2026-11-03.
  3. Ask for a written repair-first scope for historic mullions, with replacement in kind identified only where repair is not possible. Source: District of Columbia Historic Preservation Office, read 2026-11-03.
  4. For historic replacement review, request drawings that label scale, measurements and component materials, so the proposed assembly can be compared on paper. Source: National Park Service, read 2026-11-03.
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What is the difference between a mullion and a muntin?

Muntins subdivide the glass-holding sash; use that distinction when describing a damaged piece in a written scope. “Mullion: A vertical primary framing member that separates paired or multiple windows within a single opening.” Source: New York City Landmarks Preservation Commission, read 2026-11-03. “Muntin: A tertiary framing member that subdivides the sash into individual panes, lights or panels.” Source: New York City Landmarks Preservation Commission, read 2026-11-03. “Note: Grids placed between two sheets of glass are not considered muntins.” Source: New York City Landmarks Preservation Commission, read 2026-11-03.

What is the difference between a mullion and a transom?

Terminology depends on the document: NYC names a horizontal bar, while the DC historic-window guide also describes the upper window itself. “Transom bar : A horizontal element that subdivides an opening, usually between a door and window.” Source: New York City Landmarks Preservation Commission, read 2026-11-03. “Transom is a small window located above a window or door, separated from the underlying opening by a wood or masonry mullion.” Source: District of Columbia Historic Preservation Office, read 2026-11-03.

What is a mullion made of?

For historic windows, the DC guide describes timber dividers and also names masonry below an upper window, so identify the existing material before ordering. “A mullion is the wood dividing member separating two abutting windows, such as between two paired windows or a transom above a window.” Source: District of Columbia Historic Preservation Office, read 2026-11-03. See the District of Columbia Historic Preservation Office passage quoted above.

How can I recognise a mullion?

On a historic window, look beyond the divider’s position and photograph its outline for the supplier to reproduce. “A mullion often has stylistic qualities, such as decorative fluting, a turned column, or a chamfered edge.” Source: District of Columbia Historic Preservation Office, read 2026-11-03. “Exterior window framing elements should be retained or replicate the size, profile and dimensions of the original condition.” Source: District of Columbia Historic Preservation Office, read 2026-11-03.

What should be checked before mullion repair?

Plan retention first for a historic assembly, and request measured drawings so the proposed work can be compared with what is there. “Mullions should be maintained and repaired; if repair is not possible, they should be replaced in kind.” Source: District of Columbia Historic Preservation Office, read 2026-11-03. “Removing a mullion and installing two paired windows without a new mullion is not appropriate, as it changes the proportions of the windows.” Source: District of Columbia Historic Preservation Office, read 2026-11-03. “Scale should be provided, measurements noted, and materials indicated for the main components of the window.” Source: National Park Service, read 2026-11-03. “Drawings of the existing historic window should be accurate, based on field measurements.” Source: National Park Service, read 2026-11-03.

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