Glossary - M

A Glossary of window terminology

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Masonry opening

The space in a masonry wall left open for windows or door. 

Mortise

A slot or rectangular cavity cut into a piece of wood to receive another part. 

Mortise-and-tenon

A strong wood joint made by fitting together a mortise in one part and a matching projecting tenon in the other. Used as a corner joint in Pella sashes. 

Mullion

A wood, metal or vinyl part used to structurally join two window or door units. 

Muntin

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

Masterframe
The combination of the head, sill and jamb sections of the full outside frame of a window.

Meeting rail
The part of a sliding glass door, a sliding window or a hung window where two panels meet and create a weather barrier.

Miter
To make a diagonal cut, beveled to a specific angle (usually 45°). Sometimes miter cuts are made into an overlapping siding or soffit panel surface, to provide a neater appearance.

Mylar
A weatherstripping material that is present where the sash frame meets the masterframe. Adds increased resistance to air infiltration.

Millwork
A term used to describe products that are primarily manufactured from lumber in a planing mill or woodworking plant.  

Molded (also Moulded)
Refers to something worked into a form or shape.  

Moulding
A strip of wood, usually shaped to a curved profile, used to accent and emphasize the ornamentation of a structure and to conceal surface or angle joints.  

Mulled Unit
Two or more window units structurally joined together.