Masterframe: The combination of the head, sill and jamb sections of a window.

Masonry openings: The openings in a masonry wall left open for windows or doors unit, the same as a rough opening in a frame wall.

Member: Any structural part of a window, such as a rail, stile, or lintel.

Mullion: A wood or metal member used to structurally join two window or door units.

Multi-Lite Sash – A sash divided into many lites.

Muntin: short or light bar, either vertical or horizontal, used to separate glass in a sash into multiple lights. Also called a grille or windowpane divider.