The parts of a window fall into three groups: the frame that anchors the unit in the wall, the sash that holds the glass, and the hardware and seals that let it open, lock and keep weather out. Knowing the names (jamb, sill, sash, muntin, balance) makes it easier to describe a problem to a contractor and to compare replacement quotes. Over the life of a window, the moving and sealing parts usually wear out long before the frame does.
The Window Frame
The frame is the fixed box that sits in the wall opening and carries the weight of everything else. It has four sides, and each one has its own name.
Head, Jambs and Sill
The head, also called the head jamb, is the top of the frame. The side jambs run up each side, and on single-hung and double-hung windows they hold the tracks the sash slides in. The sill is the bottom of the frame on the exterior. It slopes slightly outward so rainwater runs away from the house instead of sitting against the glass.
Inside the house, the flat ledge people set plants on is usually the stool, not the sill. The trim piece below the stool is the apron. The two names get mixed up constantly, and the difference matters when you describe water damage: a stain on the interior stool can point to a problem at the exterior sill or the flashing above the window.
Casing and Drip Cap
Casing is the trim that surrounds the window and covers the gap between the frame and the wall. Interior casing is mostly decorative. Exterior casing, often called brickmould when it has a thick profile, also protects that joint from rain and wind. Above the head, a drip cap (a narrow strip of flashing) pushes water outward so it doesn't run down the face of the window.
Rough Opening and Nailing Fin
The rough opening is the framed hole in the wall before a window goes in. New-construction windows usually have a nailing fin, a flange around the outside that fastens to the wall sheathing and gets covered by siding. Many replacement windows are built to fit inside an existing frame instead, so they have no fin. That difference is one reason quotes for a full-frame replacement and an insert replacement can look so different.
The Sash
The sash is the part of a window that holds the glass. On windows that open, it's the piece that moves. A picture window has no operable sash at all, because the glass is set directly into the frame.
Rails and Stiles
The horizontal pieces of a sash are rails, and the vertical pieces are stiles. On a double-hung window, the bottom rail of the upper sash and the top rail of the lower sash meet in the middle at the meeting rail, sometimes called the check rail. That's where the lock sits, and it's where drafts creep in once the two sashes stop lining up.
Glazing and Insulated Glass
Glazing is the glass itself, along with the way it's set into the sash. Most modern windows use an insulated glass unit: two or three panes separated by a spacer around the edge and sealed together. The space between the panes is often filled with argon or krypton gas, which slows heat transfer better than air. Many units also carry a low-E coating, a thin metallic layer that reflects heat while letting visible light through.
When the edge seal on that unit fails, moisture gets between the panes and the glass stays hazy no matter how often you clean it. That's a glazing problem, not a frame problem.
Muntins, Grilles and Mullions
These three get confused more than any other window parts, so here is the short version:
- Muntins are the strips that divide the glass within a single sash into smaller panes, called lites.
- Grilles create the look of divided lites with decorative inserts, either sealed between the panes of an insulated unit or attached to the glass surface.
- Mullions are the structural pieces that join two separate window units side by side or stacked.
A row of three windows set together has mullions between them. A colonial-style grid across one sash is made of muntins or grilles.
Window Hardware and Weather Seals
Hardware is what lets a window open, close and lock. It's also where most day-to-day wear happens, because these are the parts you touch every time you use the window.
Locks, Keepers and Tilt Latches
On hung windows, the sash lock (often a cam lock) mounts on the meeting rail and turns into a keeper on the opposite sash. As it locks, it pulls the two sashes together and tightens the seal between them. Many double-hung windows also have tilt latches at the top of the lower sash, so it can tip inward for cleaning from inside the house.
Balances
Balances are the spring or counterweight systems hidden in the side jambs of single-hung and double-hung windows. They hold the sash wherever you leave it. When a balance wears out, the sash creeps down, slams shut or refuses to stay open.
Cranks, Operators and Hinges
Casement and awning windows open on hinges instead of sliding in a track. A crank handle drives an operator and arm that swings the sash outward. Casements hinge on the side, and awning windows hinge at the top. A stripped crank or a bent operating arm is a common reason these windows stop opening all the way.
Weatherstripping and Screens
Weatherstripping is the flexible seal where the sash meets the frame, and it blocks both air and water. Casement and awning windows press the sash firmly against that seal when they close, which is why they tend to seal tightly. Screens sit in their own lightweight frame on the outside or inside of the window, depending on the style.
How Window Parts Change by Style
Every operable window shares the same basic frame parts, but the sash and hardware change from style to style:
- Single-hung windows have a fixed upper sash and a sliding lower sash, with balances in the jambs and one lock.
- Double-hung windows let both sashes slide, and most add tilt latches so each sash tips in for cleaning.
- Casement windows use one side-hinged sash, a crank operator and compression weatherstripping.
- Awning windows work like a casement turned on its side, hinged at the top with a crank.
- Picture windows hold fixed glass in the frame, so there are no balances, cranks or locks to wear out.
Which Window Parts Wear Out First
A window frame usually outlasts the parts attached to it. Weatherstripping compresses, cracks or pulls loose, and you notice it as a draft or a whistle on windy days. Balances lose tension, so hung sashes won't stay put. Crank operators strip or bind. Insulated glass seals fail and leave fog between the panes.
A single worn part on an otherwise sound window is usually worth fixing, since balances, weatherstripping, locks and operators are made as replaceable components. Replacement starts to make more sense when the frame itself is soft, warped or rotted, when several glass units have lost their seals, or when parts for an older window are no longer made. If you're weighing the two, write down which part is failing using the names above. It makes the conversation with a contractor faster and the quote more accurate.
Getting the Right Window for Your Home
Forsyth Exterior Contractors installs single-hung, double-hung, casement, awning and picture windows, along with French, sliding and patio doors. See the options on our window and door installation services page, then book a free consultation to point out the parts giving you trouble and get upfront pricing before any work begins.
