FrameThe outer structure that holds the sash in position. It is the head jamb across the top, a side jamb down each side and the sill along the bottom.
Casing (interior trim)Flat, decorative molding around the inside perimeter of a window. It covers the space between the unit and the rough opening in the wall. Trim like this is fitted around a window and is not strictly part of it.
Sash lock (cam lock)A rotating lock mounted at the check rails. Turned closed, it catches a keeper on the other sash, pulls the two sashes tight together and holds the window shut. Larger windows use two.
Glazing (glass)The glass in a sash, and also the act of installing it. Most new windows use insulating glass: two or more panes sealed together around a spacer, as the section on the right shows.
SashThe part of a window holding the glass. It opens and closes. Stiles, rails and glass joined together. This window has two, and both slide. A sash that does not open is called fixed.
Outside view 7
Drip cap (head flashing)A formed piece installed at the top of a window or door. Made of aluminum or vinyl. It sends water out over the face of the trim so that it cannot seep in behind the top of the unit.
Nailing fin (mounting flange)A thin strip around the perimeter of the frame. It is fastened to the wall sheathing to secure the unit, and adds protection against water getting in. In the United States a window sold for new construction has one.
Grille (muntin bars)Bars dividing the glass into a grid of small panes, called lites. A true muntin separates and holds individual panes. A grille gives the same look with bars on the surface of one large pane, or sealed between two panes.
Brickmould (exterior casing)Exterior trim around the frame that butts against the brick or siding. It hides the gap between the frame and the wall surface. It is often fitted at the factory, and it was traditionally the piece used to attach the window to the wall.
Insect screenA tightly woven mesh of metal or fiberglass in its own frame. It lets air through and keeps insects out. A half screen, like the one drawn, covers only the lower sash area.
SillThe horizontal member forming the frame's bottom. Its outer edge tapers downward so that water runs off, and it projects past the wall so that drops fall clear. In British English it may be spelled cill.
Weep holeA small hole on the outside of a window or door. It lets water drain to the outside.
Inside view 10
Head jambThe horizontal member across the top of the frame. Jamb is the name for the frame members at the top and sides, so a window has one head jamb and two side jambs.
Top railThe uppermost rail, across the top of the upper sash. Rails are the horizontal members of a sash, and stiles are the vertical ones.
Jamb extension (extension jamb)A piece, usually wood, fastened to the inside edge of the frame. It extends the frame to the full depth of the wall and covers the space between the frame and the finished interior wall. It is not needed where drywall runs up to the frame.
Side jambThe vertical member that forms one side of the frame. There is one on each side, and on a hung window the two sashes slide up and down between them.
Check railHorizontal sash members that meet in the middle of a hung window. They are the bottom rail of the upper sash and the top rail of the lower sash. The lock and its keeper are mounted here.
StileThe vertical members of a sash. Rails are the horizontal ones. Each sash has a stile on the left and one on the right.
Sash liftA handle on the bottom rail of a hung window. It makes the sash easier to raise and lower.
Bottom railThe lowest horizontal member of the lower sash. It carries the sash lift, and when the window is shut the weatherstripping under it seals it to the sill, as the section shows.
Stool (interior sill)A flat interior trim piece at the bottom of the window. It laps over the sill, projects into the room and runs out past the casing on both sides. Carpenters call it a stool, though many people call it the windowsill.
ApronA piece of casing or ornamental trim immediately beneath the stool. Installed flush against the wall.
Section through the glass 5
Low-E coatingA microscopically thin layer of metal or metallic oxide on the glass. It is virtually invisible and suppresses the flow of radiant heat. In a two-pane unit it is usually on one of the two glass faces inside the sealed space.
Air space (gas fill)The sealed space between the panes of insulating glass. It is often filled with argon, a colorless, odorless gas that conducts about two thirds as much heat as air does.
Glazing beadA strip that holds the glass in place in the sash. Made of profiled wood or vinyl. It covers the space between the edge of the glass and the sash.
SpacerThe bar between the two panes of insulating glass. It seals the space between them. It usually holds a desiccant that takes up trapped moisture. Metal conducts heat, so warm-edge spacers use other materials.
WeatherstrippingA resilient, compressible strip that seals the sash to the frame. It cuts air and water leaks. It was once a soft metal such as brass. Today it is usually silicone, rubber, vinyl or foam.
How this list was compiled: Part names and every detail line were checked, for facts only and with nothing copied, against five kinds of page read on 2026-09-19. Two window makers' glossaries: https://www.marvin.com/support/glossary (frame, head jamb, side jamb, sill, sash, stiles, rails, check rails, glazing bead, spacer, Low E glass, argon, drip cap, brick mould casing, jamb extension, stool, apron, sash lock, screens, weatherstripping, mullion) and https://www.andersenwindows.com/support/window-door-glossary (frame, jamb, sill, sash, stile, rail, check rail, keeper, lift, glazing, light or lite, insulating glass unit, glazing bead, grille, casing, brick mould, extension jamb, stool, apron, drip cap, nailing flange, weep holes, insect screen, half insect screen, insert window, full frame, rough opening, header). The same two makers' explainers: https://www.marvin.com/blog/parts-of-a-window (nailing fin, cam lock, weather-stripping materials, tapered sill, glazing) and https://www.andersenwindows.com/ideas-and-inspiration/blog/tips/parts-of-a-window (trim is not strictly part of a window). Wikipedia: https://en.wikipedia.org/wiki/Window (new-construction and replacement windows in United States usage, single-hung and double-hung sashes), https://en.wikipedia.org/wiki/Window_sill (sloping and projecting sill, the spelling cill, stool and windowsill), https://en.wikipedia.org/wiki/Muntin (muntin, glazing bar, muntin against mullion, the cost of small panes before the mid 19th century) and https://en.wikipedia.org/wiki/Insulated_glazing (spacer and desiccant, warm-edge spacers, argon at 67 percent of the thermal conductivity of air, Low E coatings on the second or third glass surface). Weep holes rest on the Andersen glossary alone. Labels, hints, detail lines, groups and position notes were written for this site. Licence: own work
The drawing: Drawn for this site, 2026-09-19. Own original line drawing, nothing traced or copied. Read for accuracy: the Wikipedia articles Insulated glazing, Sash window and Window sill, and the part list and layout brief in this dataset. Licence: own work
How it works
This parts of a window diagram shows one house window three ways: from outside, from inside, and cut through its bottom edge. It has 27 numbered parts. Open Study to look a name up, or quiz yourself in Find it or Name it. Print blank makes a numbered sheet.
Read the full guide
Exterior parts of a window, then the interior
The Outside view set zooms to 8 parts. Four are parts of a window frame exterior: the frame, the sill, the weep hole and the nailing fin, which shows where siding is peeled away. Inside view holds the 14 parts of a window interior, from head jamb to apron. Section through the glass holds the last 5, from Low-E coating to weatherstripping.
Parts of a window trim, frame and sash
Four more sets sort the same 27 by job: Frame 7, Sash and glass 11, Trim and flashing 5, and Hardware, seals and screen 4. Sources disagree on trim. One manufacturer's glossary counts the brickmould as a frame member, and another says trim is not part of a window. The quiz asks for all 5 trim names either way. A wrong tap shows the right part. Miss four, and Practise the 4 you missed asks them again.
Muntins divide the glass inside one sash, and the quiz takes muntin for the grille. A mullion joins two separate window units, so it is not drawn.
The sill is the sloped bottom of the frame, outdoors. The stool is the flat shelf indoors. Under Name it, Type it takes windowsill for the sill only.
Rails run across a sash and stiles run up its sides. Where two sashes meet, the rails are check rails, and the sash lock sits there.
Questions and answers
What are the 5 parts of a window?
The frame, the sash that holds the glass, the glazing, the casing around the opening and the sash lock. Lists differ, so treat these five as a start. The Start here: 5 main parts set asks only these.
Are the parts of a replacement window different?
No. In the United States a replacement window is a framed unit that slips inside the old frame from indoors after the old sashes come out. A new construction window has a nailing fin and goes in from outside before the siding. The parts of a vinyl window match too: frame material does not change the names.
Does every window have all 27 parts?
No. The drawing is a composite, so most parts of a home window appear once. A nailing fin and a brickmould are two ways of fixing a window to a wall. A casement or picture window has one sash and no check rail.