The vertical side member of a doorway, window, fireplace, or other architectural opening; in carved portals, jambs often serve as supports or settings for sculpture.
A jamb is one of the vertical sides of a doorway, window, fireplace, or other architectural opening. In stone architecture, the jamb may be formed from one or more dressed blocks and supports the lintel, arch, or other construction above.
Jambs are particularly important to architectural sculpture because they often provide the structural and visual setting for carved ornament or figures. In Romanesque and Gothic portals, sculptors frequently adapted standing figures to the tall, narrow proportions of the jamb.
Etymology
From Old French jambe, “leg,” reflecting the idea of the upright side or supporting member of an opening.
Sculptor Notes
For the sculptor, a jamb is not merely an architectural boundary. Its proportions can govern the stance, scale, silhouette, and degree of projection of a figure designed for a portal.
References
https://www.pierres-info.fr/glossaire/index.html