Masonry opening
Actual block opening – not including the buck strips.

Meeting rail
(Lock rail) – One of the two horizontal members of a double-hung sash which come together. See: Check rail.

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

Window sash and other wood products made in a wood-working plant.

Miter joint
Two members joined at an angle, commonly 45 degrees.

Size in width and height are given in feet and inches to even sizes. Some modular sizes use a combination of modular width and commodity heights.

Moisture barrier
(Vapor barrier) – A material which retards the passage of water vapors from one space to another. Polyethylene sheet is commonly used as a vapor retarder.

Moisture content
Percentage of dry weight of material which is composed of water, such as in wood.

(Molding) – A relatively narrow strip of wood used to conceal a joint or to emphasize ornamentation of a structure.

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

(Moulding) – British spelling of mold, and molding.

Mull clip
A clip designed to anchor a tube mullion in an opening.

Mullion / Mull
Extrusion that connects two or more windows or doors together in a single opening.

Multi-lite sash
A sash divided into many lites.

Multi-point lock
A lock with multiple locking points, typically to the head and sill, jamb or astragal.

Exterior and interior extrusions or wood in the sash of a window or door which appear to divide the glass into smaller lites.