What are the parts of a window called?
Frame, sash, mullion, transom, cill and bead, plus the ironmongery — stay, fastener, friction hinge and trickle vent. What each part is and which ones you can actually replace.
- Difficulty
- beginner
- Time
- 8 min
- Read
- 5 min
- Safety
- caution
Short answer
The fixed outer part is the frame. An opening part is a sash or a casement. A vertical divider within the frame is a mullion and a horizontal one is a transom. The cill throws water clear at the bottom, the bead holds the glass in, and the stay, fastener and hinges are the ironmongery — which is usually replaceable independently of the window.
Windows are the part of a house where the vocabulary is most worth having, because so much of what goes wrong is a small replaceable component, and finding the right one depends entirely on being able to name it and identify the system it belongs to.
Safety
Step by step
- Frame and sash or casement.The frame is fixed into the opening. A casement is an opening light hinged at the side or top. A sash, in a sash window, slides vertically. Both words are used loosely for any opening part, and both are common on suppliers' sites.
- Mullion and transom.A mullion is a vertical member dividing a window into lights; a transom is a horizontal one. The small window above a transom is often fixed, and the transom is a common location for a trickle vent.
- Cill, drip and drainage.The cill at the bottom throws water clear of the wall below and has a drip groove on its underside. In a uPVC or aluminium frame there are also drainage slots along the bottom of the outer face, which let out the water that gets past the seals. Both need to be clear.
- Glazing bead and gasket.The bead is the strip holding the glass or the sealed unit in the rebate — internal on modern windows, which is why glass cannot simply be removed from outside. The gasket is the rubber seal between the bead and the glass.
- Ironmongery on a casement.The friction stay is the hinge arm that holds the window at any angle; the fastener or cockspur is the handle that pulls it shut; the stay peg or restrictor limits how far it opens. All three are replaceable and all three are system-specific.
- Ironmongery on a sash window.Cords or chains running over pulleys to counterweights in the boxes each side, or spiral balances in a modern equivalent, plus a sash fastener at the meeting rail and often stops or restrictors. The meeting rail is where the two sashes overlap.
- Trickle vents.The controllable slot vent in the frame or the head, providing background ventilation independently of opening the window. They are required in many replacement situations and they are the single most commonly taped shut component in a house.
- Identify the system before buying anything.Hinges, stays, handles and locks are made to fit particular systems, and there are many. Photograph the part, measure the hinge stack height and the screw hole spacing, and look for a stamp on the metalwork.
Tips
- Clean the drainage slots along the bottom of uPVC frames once a year. Blocked slots let water stand in the frame, which is one of the main causes of early sealed unit failure.
- The stamp on a friction stay or a lock mechanism is usually the fastest route to the right replacement. It is often on the moving arm rather than the fixed part.
- A window that has to be lifted or pushed to lock is usually a hinge that has sagged, not a lock that has failed.
Common mistakes
- Taping over trickle vents to stop draughts — They are the background ventilation the house was designed with, particularly in a house with sealed windows and no other provision. Closing them raises humidity and produces condensation and mould at cold surfaces.
- Fitting a keyed restrictor to a bedroom or loft escape window — An escape window has to be openable without hunting for a key. Fitting a lock to one can turn a survivable fire into a fatal one, and it is the reason escape windows are identified in the design.
- Trying to remove a sealed unit from outside a modern window — The beads are internal by design, precisely so the glass cannot be removed from outside. Prising at the external gasket damages the frame and achieves nothing.
If it doesn't work
The casement drops and catches on the frame when closing
Cause: A worn or bent friction stay — Fix: Replace the stay. It is an inexpensive, system-specific part and it is a far better answer than easing the frame, which cannot be undone.
Water pooling inside the bottom of the frame
Cause: Blocked external drainage slots — Fix: Clear the slots from outside with a thin plastic probe. Do not enlarge them and do not seal them.
A sash window that will not stay up
Cause: A broken sash cord, or a spiral balance that has lost its tension — Fix: Both are replaceable. On a traditional box sash it means opening the pocket in the frame and rebalancing; it is a well-established repair and much cheaper than replacement.
Questions people ask
What is a meeting rail?
On a sash window, the pair of rails where the top and bottom sashes overlap in the middle. It is where the fastener goes and where draughts are worst on an unrestored window.
What is a window board?
The internal shelf at the bottom of the opening, sitting on the inner leaf of the wall. It is not the cill — the cill is the external component that throws water off.
Which windows have to open for escape?
It depends on the age and layout of the house, but loft conversions and habitable rooms in newer houses commonly rely on a designated escape window with a minimum openable area and a maximum cill height. It is worth knowing which yours are.
Can I replace just the ironmongery?
Usually yes, and it is often the sensible repair. Stays, handles, locks and gaskets are all available separately, and identifying the system correctly is the whole of the difficulty.