Windows and doors glossary

You’ll be more comfortable shopping for windows and doors once you know the lingo,so this handy glossary (in alphabetical order) will help you make sense of it all. Your Isothermic advisor can answer any other questions you have. We’ve been in the business for over 40 years.


A structure that supports the window and keeps its sashes in place.


A structure that includes the sill or threshold, jambs, and top rail.


Vertical parts of a window or door casing.

Lintel (or header)

A horizontal beam that forms the top of the window/door frame.


A vertical or horizontal frame member that divides the panes of a window.

Extruded insect screen

An all-aluminum screen with no plastic corners that eventually yellow or break.

Glass pane

A sheet of glass that is cut and inserted in a window.

Fenestration product

Window or door.


A horizontal piece that forms the bottom of a window.

Thermica threshold

Isothermic-exclusive high performance threshold system.

Faux mullions

Mullions that are permanently applied to the exterior and interior sides of the glass to divide the window into small sections.

Top rail

Horizontal member along the top of the casing.


The composition of glass inserted in a window (single, double, triple, sealed unit, safety glass, tempered glass, etc.).

Tempered glass

Glass that is toughened to be two to five times stronger than regular glass.


A member inside the frame that holds the glass in place.