provide passive fire protection. A delay in the spread of fire gives occupants more time to escape and fire fighters more time to extinguish the fire. Some fire walls allow fire resistive window assemblies, and are made of non-combustible material such as concrete, cement block, brick, or fire rated drywall. Wall penetrations are sealed with fire resistive materials. A doorway in a firewall must have a rated fire door. Fire walls provide varying resistance to the spread of fire, (e.g., one, two, three or four hours). Firewalls can also act as smoke barriers when constructed vertically from slab to roof deck and horizontally from an exterior wall to exterior wall subdividing a building into sections. Shear wall Shear walls resist lateral forces such as in an earthquake or severe wind. There are different kinds of shear walls such as the steel plate shear wall. Knee wall Knee walls are short walls that either support rafters or add height in the top floor rooms of houses. In a -story house, the knee wall supports the half story. Cavity wall Cavity walls are walls made with a space between two "skins" to inhibit heat transfer. Pony wall Pony wall (or dwarf wall) is a general term for short walls, such as: A half wall that only extends partway from floor to ceiling, without supporting anything A stem wall—a concrete wall that extends from the foundation slab to the cripple wall or floor joists A cripple wall—a framed wall from the stem wall or foundation slab to the floor joists Demountable wall Demountable walls fall into 3 different main types: Glass walls (unitesed panels or butt joint), Laminated particle board walls (this may also include other finishes, such as whiteboards, cork board, magnetic, etc., typically all on purpose-made wall studs) Drywall Solar energy A trombe wall in passive solar building design acts as a heat sink. Shipbuilding On a ship, a wall that separates major compartments is called a bulkhead. A thinner wall between cabins is called a partition. Boundary wall Boundary walls include privacy walls, boundary-marking walls on property, and town walls. These intergrade into fences. The conventional differentiation is that a fence is of minimal thickness and often open in nature, while a wall is usually more than a nominal thickness and is completely closed, or opaque. More to the point, an exterior structure of wood or wire is generally called