Manhole:

A large chamber sunk into the ground, allowing physical access to underground drains.

Mansard roof:

A steeply sloping, almost vertical roof often containing windows, housing accommodation within the roof space, usually with a flat or shallower roofed section above.

Masonry:

A general term used for stones, bricks or concrete blocks in an external wall, bonded together with mortar.

Mastic:

A flexible and waterproof sealant, often used to fill small gaps around window or door frames, or applied to the outer face of an expansion joint.

Mortar:

A mixture of sand, cement and/or lime, used to bind together bricks, stones or other masonry.

Mullion:

A vertical division between two windows or at the corners of a bay window, often of stone construction.

Mullion:

A vertical member of a frame dividing two window panes, or a more substantial vertical division between two separate windows, e.g. in a bay.