---
title: "What Is Mullion? Window and Door Glossary | Honey Rock Window and Door"
description: "A mullion is the vertical or horizontal bar that joins two window or door units side by side or stacked."
url: https://honeyrock.multisites.ai/glossary/mullion
publisher: Honey Rock Window and Door
phone: +1-214-484-9982
quote: https://honeyrock.multisites.ai/contact-us
---

# Mullion

A mullion is the vertical or horizontal bar that joins two window or door units side by side or stacked.

A mullion ties separate units into one larger opening. Think of a pair of windows set next to each other, or a door with a window stacked above it. The mullion carries the joint and often adds strength.

Do not confuse it with a [muntin](https://honeyrock.multisites.ai/glossary/muntin). A muntin divides glass inside a single [sash](https://honeyrock.multisites.ai/glossary/sash). A mullion divides whole units. Mullions affect the look of a wall, so ask for the visible width early. Slimmer mullions keep more view, a point we cover in the [sightline](https://honeyrock.multisites.ai/glossary/sightline) entry and across our [steel windows and doors](https://honeyrock.multisites.ai/steel-windows-and-doors). Ask whether the mullion is structural.

[Where it mattersSteel windows and doors](https://honeyrock.multisites.ai/steel-windows-and-doors)

## Related terms

-   [Muntin](https://honeyrock.multisites.ai/glossary/muntin)
-   [Sash](https://honeyrock.multisites.ai/glossary/sash)
-   [Sightline](https://honeyrock.multisites.ai/glossary/sightline)
-   [Jamb](https://honeyrock.multisites.ai/glossary/jamb)

![A modern home with wide glass doors lit warm at dusk beside a pool](https://honeyrock.multisites.ai/_astro/dusk-modern-home-pool-01.CeePxgPr_Z2giQin.webp)

## Let’s frame your view.

Send your plans or a few photos of the openings. We will help you choose the system, the brand and the glass.

[Start your project](https://honeyrock.multisites.ai/contact-us)[or call (214) 484-9982](tel:+1-214-484-9982)