A jamb extension does precisely what the name suggests – it's a board that fills the depth of the wall space from the inside face of the door frame to the face of the wall and thus widens the frame. So if the wall thickness is greater than the standard jamb width, extensions must be used.

Gezit jamb extensions are typically made of MDF and create a clean edge for the casing trim to be nailed to or inserted if this is a telescopic L-casing.

With the help of extensions Gezit frames can cover walls 4 7/8” to 6 7/8” thick.