Understanding the parts of a window can make shopping for replacement windows or planning a window replacement much easier. Each component plays a role in the window’s appearance, performance, functionality, and energy efficiency. Knowing the correct terms helps you compare style options, understand how windows work, and communicate clearly with your installer.
Whether you’re looking at traditional double-hung windows, casement windows, sliding windows, or a single-hung window, most share the same basic window components. This guide walks you through the key parts, so you can recognize each one and understand its purpose.
Overview of Window Parts
Windows have several sections, each with specific components that work together for structure, function, and style. While not all windows have the same features, most include these core areas:
- Frame: The outer structure that holds and supports the entire window.
- Sash: What holds the window glass and can be fixed or movable.
- Glass and glazing: The panes, which can have single-pane, dual-pane, or multiple panes of glass separated by air space for insulation.
- Hardware: The handles, locks, and other moving parts that allow a window to open, close, and lock.
- Seals and weather protection: These features, like weather stripping, keep out drafts, moisture, and debris while helping to reduce heat transfer.
- Decorative elements: The grilles, interior trim, and other style details that enhance curb appeal.
Window Frame Parts
The frame is the outermost structure of the window. It forms the connection between the window unit and your home’s wall, holding the sash and window pane securely in place. A sturdy frame ensures the window operates smoothly, seals properly, and stands up to the elements for years to come.
Head
The head is the top horizontal section of the frame. It spans the width of the window opening and helps support the weight above it. In many designs, the head also contains tracks or channels that guide moving sashes and two or more windows installed side by side.
Sill
The sill is the bottom horizontal section of the frame. It plays a critical role in shedding water away from the window to prevent leaks. Quality sills are often sloped or equipped with drainage features to keep moisture from entering the home. Some designs incorporate a window stool, which is the flat interior ledge or shelf at the base of the window, often used for decoration or small displays.
Side Jambs
The side jambs are the vertical sides of the frame. They house the channels, balance systems, or hinges that allow the sash to open and close. Properly aligned window jambs ensure smooth operation and a tight seal when the window is shut.
Mullion
A mullion is a vertical element that separates two or more windows in a multi-window frame. It can be structural, supporting the frame between windows, or decorative to achieve a certain design style.
Jamb Extensions
Jamb extensions increase the depth of the window frame, so it matches the thickness of the surrounding wall. They create a seamless transition between the window and the interior wall surface.
Brickmould
Brickmould is an exterior trim piece that surrounds the outside of the frame. It covers the gap between the window and the siding or masonry, while adding a finished look to the exterior, and serves as low-maintenance trim for modern windows.
Casing
Casing is the trim that finishes the perimeter of the window frame on the inside or outside of the home. It hides installation gaps and provides a clean, decorative border. Interior trim options can be simple or elaborate to match your home’s style.
Apron
The apron is a horizontal trim piece located below the interior sill on the inside of the home. It visually frames the window and can add to the room’s architectural style.
Drip Cap
The drip cap is a small flashing piece installed above the head of the frame. Its job is to direct water away from the window, reducing the risk of leaks and water damage.
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Window Sash Parts
The sash is the part of the window that holds the window glass in place. Depending on the type of window, the sash can be fixed or movable. A well-built sash contributes to energy-efficient performance, security, and ease of use.
Upper Sash
The upper sash is the top section of a double-hung window. In many designs, it can slide downward to allow airflow from the top, which helps with ventilation while keeping the lower section closed. On a single hung window, the upper sash is fixed.
Lower Sash
The lower sash, sometimes called the bottom sash, is the bottom section of a double-hung window. It typically slides upward to open, making it the most commonly used sash for day-to-day ventilation. The lower sash must align and seal tightly against the meeting rail of the upper sash to prevent drafts and water intrusion.
Rails
Rails are the horizontal sections of the sash. The top rail runs along the upper edge of the sash, and the bottom rail forms its lower boundary. Rails contribute to the sash’s strength and also hold weather stripping that improves energy efficiency and can help reduce heat transfer.
Stiles
Stiles are the vertical sides of the sash. They help define the sash’s shape, add rigidity, and provide a mounting point for hardware. In operable windows, the stiles also guide the sash’s movement within the frame.
Meeting Rail
The meeting rail is where the upper and lower sashes come together in the closed position on a double-hung window. This joint is designed to lock securely, seal tightly, and align the two sashes.
Grilles or Muntins
Grilles, also called muntins, are decorative or functional dividers that create the look of individual panes within a single sash. They can be placed on the glass surface, set between multiple panes, or designed as true divided lites. Variations include full divided light, simulated divided light, and decorative bars to match specific style preferences.
Fixed Sash
A fixed sash is non-operable and is typically found in single-pane windows, new windows with picture designs, or as part of a configuration with two or more windows where only some are operable.
Glass and Glazing Parts
The glass is the central feature of any window, allowing natural light into your home while providing a view of the outdoors. Modern window components go beyond simple single-pane glass to significantly improve comfort and reduce heat transfer.
Single, Double, or Triple Pane Glass
Windows can have single-pane, dual-pane, or multiple glass panes. Double- and triple-pane designs improve insulation by creating air space between the panes. More panes generally mean better energy-efficient performance and noise reduction.
Low-E Coating
Low-emissivity (Low-E) coatings are microscopically thin layers applied to the glass surface. They reflect heat toward its source, helping keep your home warmer in winter and cooler in summer. Low-E coatings can also reduce UV damage to floors, furniture, and fabrics.
Gas Fills
In many double- and triple-pane windows, the space between panes is filled with an insulating gas, such as argon. These gases are denser than air, slowing heat transfer and improving performance.
Warm-Edge Spacer
The spacer is the material that separates the panes of glass at their edges. Warm-edge spacer bars are made from materials that conduct less heat, improving insulation and reducing condensation along the edges of the window pane.
Edge Seal
The edge seal keeps the panes securely bonded to the spacer while preventing moisture from entering the space between panes. A high-quality edge seal is critical to preventing fogging and maintaining insulation performance over time.
Grilles Between Glass
Some windows feature decorative grilles placed between individual panes of glass. It creates the look of full divided light or simulated divided light designs without adding surfaces that need cleaning, offering both style and low-maintenance convenience.
Decorative Glass Options
Some windows feature glass with etched patterns, frosted finishes, or stained designs for added style and privacy. These style options allow homeowners to customize the look of their windows without sacrificing natural light or easy cleaning.
Window Hardware Parts
Hardware includes the components that allow a window to open, close, and lock securely. These parts are essential for ease of use, home security, and weather protection. While hardware styles can vary by window type, most fall into a few common categories.
Locks and Latches
Locks and latches secure the window when closed, helping prevent break-ins and keeping the sashes tightly sealed. On double-hung windows, a cam lock, a rotating latch that pulls the sashes tightly together when turned, is typically located where the two sashes meet. Casement windows and awning windows often use a latch that pulls the sash snug against the frame.
Multi-Point Locks
Multi-point locks secure the sash at several points along its edge rather than just one. They are often found on large casement windows or sliding windows, providing extra security and improving the tightness of the seal.
Cranks and Operators
Casement and awning windows often use a crank handle connected to an operator arm to open and close the sash. This system allows smooth operation and precise control over how far the window opens.
Hinges
Hinges are used on casement, awning, and hopper windows to attach the sash to the frame. They allow the sash to pivot outward or inward and are designed to support the weight of the glass while ensuring smooth movement.
Tilt Latches
Tilt latches are small mechanisms on the top of a double-hung sash that release it from the tracks, so it can tilt inward. This feature makes easy cleaning possible from inside the home.
Balance System
In single- and double-hung windows, the balance system counteracts the weight of the sash to make opening and closing easier. Common types include spiral balances, block-and-tackle systems, and constant force balances.
Vent Stops
Vent stops are small mechanisms that limit how far a sash can be opened. They are often used for child safety, security, or to allow partial ventilation without fully opening the window.
Seals and Weather Protection Parts
Seals and weather protection features keep drafts, moisture, and debris out while improving energy-efficient performance. They play a critical role in maintaining indoor comfort, protecting the window, and ensuring low-maintenance operation over time.
Weatherstripping
Weather stripping is a flexible sealing material applied where the sash meets the frame. It compresses when the window is closed to block air leaks, reduce heat transfer, and improve insulation. Different types of weatherstripping are used depending on the window style and location.
Sill Pan
A sill pan is a waterproof barrier installed beneath the window sill during installation. It helps direct any water that gets past the frame outward, protecting the wall and substructure from moisture damage.
Flashing
Flashing is a thin, waterproof material installed around the window frame, usually under siding or exterior trim. It prevents water from seeping behind the frame and into the wall, which can lead to rot or mold.
Weep Holes
Weep holes are small openings at the bottom of the frame or sash that allow trapped water to drain away. They help prevent moisture buildup inside the frame, which can cause damage over time and reduce the lifespan of replacement windows.
Gaskets
Gaskets are rubber or foam seals used around the window pane or in certain parts of the frame to create a tight, weather-resistant barrier. They work alongside weather stripping to block drafts and help maintain energy-efficient performance.
Different Window Types and Their Unique Parts
While many window components are the same across designs, certain parts are unique to specific operating styles. This table outlines the key parts for each type, so you can see how functionality and structure differ.
| Window Part | Hung Windows (Single & Double-Hung) | Sliding Windows | Casement / Awning / Hopper Windows | Fixed Windows |
| Upper sash / lower sash | ✔ | ✘ | ✘ | ✘ |
| Meeting rail | ✔ | ✘ | ✘ | ✘ |
| Balance system | ✔ | ✘ | ✘ | ✘ |
| Tilt latches (in tilt models) | ✔ | ✘ | ✘ | ✘ |
| Sliding sash | ✘ | ✔ | ✘ | ✘ |
| Rollers or glides | ✘ | ✔ | ✘ | ✘ |
| Tracks | ✘ | ✔ | ✘ | ✘ |
| Interlock (where sashes meet) | ✘ | ✔ | ✘ | ✘ |
| Crank handle/operator arm | ✘ | ✘ | ✔ | ✘ |
| Friction hinge or butt hinge | ✘ | ✘ | ✔ | ✘ |
| Locking handle or lever | ✘ | ✘ | ✔ | ✘ |
| Weather-seal compression points | ✘ | ✘ | ✔ | ✘ |
| Non-operable sash | ✘ | ✘ | ✘ | ✔ |
| Fixed frame | ✘ | ✘ | ✘ | ✔ |
Every Part of Your Window Matters To Us
Knowing the names and functions of different window parts can make it easier to compare products, understand performance features, and communicate with your installer. From the frame and sash to the window glass, hardware, and specialty features, each component plays a role in how your window serves your home’s needs.
Whether you’re replacing a single window or upgrading two or more windows in your house, understanding these terms will help you make informed decisions. And when you’re ready for expert guidance, Lake Washington Windows & Doors can recommend the right options for your home and ensure professional installation backed by our Leak Armor™ installation guarantee.
