Jambs form the sides and top of a window frame. They are the vertical and horizontal supports of the window. The vertical jambs are also known as “side jambs,” and the horizontal jamb at the top of the window is called the “head jamb.” In some windows, the jambs house other pieces, such as tracks, rails, balance systems, latches, and locks.
Although the jamb provides the structural support for the window, they can also be a weak point. If not fitted securely, they can cause leaks and interfere with the security and function of a window. Well-fitted and properly secured window jambs, however, can improve the appearance and strength of a window. They can also signal a leak or shift in the window when they become ill-fitting, stained, or warped.