Transom vs. Mullion — What's the Difference?
By Tayyaba Rehman — Updated on November 4, 2023
Transom is a horizontal architectural bar above a door or between a door and window above it; mullion is a vertical bar between the panes of glass in a window.
Difference Between Transom and Mullion
Table of Contents
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Key Differences
A transom is a structural beam or bar that is situated horizontally above the doorframe or between the top of a window or door and another window above it. It often supports the structure above or serves as a base for a window. Mullions, in contrast, are vertical elements that divide windows into sections. They provide structural support to an arch or a piece of framework.
Transoms can be decorative or functional, adding both aesthetic and structural elements to a doorway or window. They are often found in residential and commercial buildings, allowing for additional light and aesthetic appeal. Mullions are primarily structural but can also be decorative. They appear in Gothic and other classic architectures, giving character and strength to large expanses of glass.
In modern architecture, transoms may be used for ventilation or to add height to a door, creating an illusion of a larger space. They can be opened or fixed and sometimes feature intricate designs. Mullions, meanwhile, are used to add complexity to window designs, contributing to both the beauty and integrity of window units. They can be made from various materials like wood, metal, or vinyl.
The use of a transom in construction dates back to the medieval period, primarily in naval architecture, later adopted in buildings. Mullions have been an integral part of window construction for centuries, especially noted in medieval church architecture. Both elements serve to enhance light and contribute to the façade’s design of a structure.
Transom windows can be seen in fanlights and over-door windows in historic buildings, signifying a higher architectural value. Mullions have a long history in stone construction, providing support for the elaborate stained glass of cathedrals and grand buildings. While the transom bears weight horizontally, the mullion does so vertically.
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Comparison Chart
Orientation
Horizontal
Vertical
Position
Above door or between door and window
Between window panes
Purpose
Structural support, decorative, allows light and ventilation
Structural support, divides window sections
Architectural Style
Common in domestic architecture, can be ornamental
Integral to Gothic architecture, often more structural
Material
Wood, glass, stone, metal
Wood, stone, metal, vinyl
Historical Use
Originally in naval architecture, later in buildings
Mainly in building architecture, especially in churches
Structural Role
Supports the weight above a door or window
Supports the window structure and divides the glass into parts
Compare with Definitions
Transom
The flat surface forming the stern of a vessel.
The ship's name was painted on the transom.
Mullion
A vertical member that provides stability to a large window.
The stone mullion was a crucial feature of the cathedral's windows.
Transom
A horizontal crosspiece over a door or between a door and a window above it.
Mullion
A vertical bar between the panes of glass in a window.
The Gothic window's mullion was elaborately carved.
Transom
(nautical) The flat or nearly flat stern of a boat or ship.a ship's transom (flat stern)
Mullion
A structural element that divides adjacent window units.
The mullion provided support for the large window facade.
Transom
A horizontal crosspiece over a door or atop a window.
The transom above the entrance was crafted with stained glass.
Mullion
A vertical divider in a series of joined windows.
The bay window’s mullion gave each pane a clear boundary.
Transom
A window above a door or another window.
Light streamed in through the arched transom.
Mullion
A mullion is a vertical element that forms a division between units of a window or screen, or is used decoratively. When dividing adjacent window units its primary purpose is a rigid support to the glazing of the window.
Transom
A small hinged window above a door or another window.
Mullion
A vertical member, as of stone or wood, dividing a window or other opening.
Transom
A horizontal dividing bar of wood or stone in a window.
Mullion
(architectural element) A vertical bar between the panes of glass or casements of a window or the panels of a screen.
Transom
A lintel.
Mullion
(transitive) To shape into divisions by means of mullions.
Transom
The stern of a square-sterned boat or ship.
Mullion
A slender bar or pier which forms the division between the lights of windows, screens, etc.
Transom
Any of several transverse beams affixed to the sternpost of a wooden ship and forming part of the stern.
Mullion
To furnish with mullions; to divide by mullions.
Transom
The aftermost transverse structural member in a steel ship, including the floor, frame, and beam assembly at the sternpost.
Mullion
A nonstructural vertical strip between the casements or panes of a window (or the panels of a screen)
Transom
The horizontal beam on a cross or gallows.
Mullion
A term often associated with both traditional and contemporary window designs.
The designer incorporated mullions for a modern take on a classic look.
Transom
(architectural element) A crosspiece over a door; a lintel.
Transom
(architectural element) A horizontal dividing bar in a window.
Transom
A transom window.
Transom
(nautical) Any of several transverse structural members in a ship, especially at the stern; a thwart.
Transom
The horizontal beam on a cross or gallows.
Transom
Items that have arrived over the transom.
We are overwhelmed with transom resumes.
They only met the deadline by working most of the night and making a transom filing.
Transom
A horizontal crossbar in a window, over a door, or between a door and a window above it. Transom is the horizontal, as mullion is the vertical, bar across an opening. See Illust. of Mullion.
Transom
One of the principal transverse timbers of the stern, bolted to the sternpost and giving shape to the stern structure; - called also transsummer.
Transom
The piece of wood or iron connecting the cheeks of some gun carriages.
Transom
The vane of a cross-staff.
Transom
One of the crossbeams connecting the side frames of a truck with each other.
Transom
A window above a door and hinged to a transom
Transom
A horizontal crosspiece across a window or separating a door from a window over it
Transom
A strengthening crossbar in a frame.
The transom added stability to the wooden frame.
Transom
An architectural element separating window segments.
They installed a transom to separate the door from the fanlight above.
Common Curiosities
Can a transom be opened?
Some transoms can be opened for ventilation, while others are fixed and purely decorative.
Are mullions structural or decorative?
Mullions are primarily structural but can also serve a decorative function.
Is a fanlight a type of transom?
Yes, a fanlight is a form of transom window, often with a semicircular or fan-like shape.
Can transoms be found on boats?
Yes, the term transom also refers to the flat surface forming the stern of a boat.
Do all windows have mullions?
Not all windows have mullions; many modern windows are a single pane without divisions.
What materials are used to make transoms?
Transoms are commonly made from wood, metal, glass, or stone.
What is the main purpose of a transom?
The main purpose of a transom is to provide structural support above a door or window and allow additional light and sometimes ventilation.
Do transoms affect the energy efficiency of a door or window?
If not properly insulated, transoms can affect energy efficiency by allowing heat transfer.
Can transoms be retrofitted into existing doors or windows?
Yes, transoms can be added to existing structures but may require significant modifications.
Is it common to have both transoms and mullions in one window design?
It's not uncommon for larger window designs to incorporate both transoms (horizontally) and mullions (vertically).
What is a mullion in architecture?
In architecture, a mullion is a vertical element that divides sections of a window.
Are mullions only used in Gothic architecture?
While prominent in Gothic architecture, mullions are used in many architectural styles.
How do mullions contribute to the safety of a window?
Mullions provide structural integrity to windows, especially in larger and divided windowpanes.
Are there different styles of mullions?
Yes, mullions come in various styles, from simple to ornate, depending on the architectural design.
Can mullions be removed for a cleaner window design?
Yes, mullions can be removed or avoided in designs aiming for a more seamless glass look.
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Tayyaba RehmanTayyaba Rehman is a distinguished writer, currently serving as a primary contributor to askdifference.com. As a researcher in semantics and etymology, Tayyaba's passion for the complexity of languages and their distinctions has found a perfect home on the platform. Tayyaba delves into the intricacies of language, distinguishing between commonly confused words and phrases, thereby providing clarity for readers worldwide.
