In this guide
The frame: head, jambs and sill
The frame is the fixed outer structure that holds the entire window in the wall opening. It has three main parts. The head is the horizontal top piece. The jambs are the vertical sides. The sill is the bottom, sloped slightly outward so water drains away from the house.
Inside the home, a flat shelf called the stool often sits at the bottom of the window, with a trim piece called the apron below it. People commonly call the stool the 'sill', but technically the sill is the exterior part of the frame.
The sash: the part that holds the glass
The sash is the frame-within-a-frame that holds the glass. On operating windows, the sash is what moves: it slides up and down on a double-hung, side to side on a slider, or swings out on a casement. A single-hung window has a fixed upper sash and a movable lower sash; a double-hung has two operable sashes.
Each sash has its own rails (horizontal pieces) and stiles (vertical pieces). Where the two sashes of a double-hung meet in the middle is the meeting rail, also called the check rail, which is where the lock is usually mounted.
Glazing: glass, spacers and seals
Glazing refers to the glass itself. Modern windows use insulated glass units (IGUs): two or three panes separated by a spacer around the edge and sealed airtight. The space between panes is often filled with argon or krypton gas for better insulation.
Warm-edge spacers made from less conductive materials reduce condensation at the glass edge compared to older aluminum spacers. When the seal around an IGU fails, moisture gets in and you'll see fogging between the panes, a problem that usually means replacing the glass unit or the window.
Glazing beads or stops are the small trim strips that hold the glass in the sash. Low-E coatings, invisible to the eye, are applied to specific glass surfaces inside the unit.
Muntins and grilles
Muntins are the strips that divide the glass into smaller panes, giving windows a traditional look. In historic windows, true divided lites used separate pieces of glass. Today, most windows use grilles that create the same look more efficiently.
Grilles may be between the glass (sealed inside the IGU, easy to clean), simulated divided lites (bars applied to both sides of the glass, often with a spacer bar between for a realistic shadow line), or removable interior grilles. The terms 'muntin' and 'grille' are often used interchangeably.
Operating hardware and balances
The hidden hardware is what makes a window easy, or hard, to use.
- Balances: springs or cords inside the jambs that hold a hung sash in place at any height
- Sash locks: secure the sashes and pull them tightly together to seal
- Tilt latches: let double-hung sashes tilt inward for cleaning
- Operators: the crank and arm mechanism on casement and awning windows
- Hinges: allow casement and awning sashes to swing open
- Window stops and limiters: restrict how far a sash opens, useful for child safety
Weatherstripping, screens and trim
Weatherstripping lines the gaps between the sash and frame to block air and water. Over time it compresses and wears out, which is a common source of drafts in older windows. Screens keep insects out and are held in a screen track or by clips.
Casing is the decorative trim that surrounds the window inside and out, covering the gap between the frame and the wall. On the exterior, brickmould or flat trim boards frame the window and integrate it with the siding. Behind the trim, proper flashing and sealant keep water out of the wall, which is just as important as the window itself.