Knowing the different parts of a window can help you select new and maintain your windows. If you’re interested in learning more about glass, check out Glass: What a Pane.

The window frame holds the window pieces together, and consists of the head, jamb, and sill.

The side jamb are the vertical pieces of the frame.

The jambliner consists of thin stripes of material the ensure a tight fit between the jamb and the sash.

The head jamb, is the horizontal piece of the frame located at the top.

The sill or stool is the horizontal piece of the frame located at the bottom.

The sash is what holds the pane(s) of glass in place. In single-hung and double-hung windows, the sash moves to open and close the window. In casement windows, the sashes swing out.

The vertical members of the sash are stiles.

The horizontal members of a sash are rails (bottom rail and top rail).

Each sash may consist of one or more planes of glass called lights or lites. If there’s just a single plane of glass in each sash, the glass pattern is one over one. If there are two lites in each sash, the pattern is two over two.

The lites are separated by mutins, sometimes called grilles, which are strips of metal or wood that separate and hold the lites in a grid.

A true divided-lite is when the mutins physically hold the panes of glass in place.

A simulated divided-lite is when the mutins are affixed to the surfaces of the window to appear as if the glass is actually divided. When the glass is insulated or there are multiple panes of glass, an optional spacer bar may be used between the panes to make the windows appear to be authentically divided.

The casing is the decorative part between the window frame and the wall.

The apron is the lower part of the casing that extends below the stool.

A mullion is a structural element that fits between two distinct but closely spaced window units. They can be horizontal or vertical.