FENESTRATION is an architectural term that refers to the arrangement@ proportion@ and design of window@ skylight@ and door systems within a building. Fenestration components include glazing material@ either glass or plastic; framing@ mullions@ muntins@ dividers@ and opaque door slabs; external shading devices; internal shading devices; and integral (between-glass) shading systems. In this chapter@ fenestration and fenestration systems refer to the basic assemblies and components of exterior window@ skylight@ and door systems within the building envelope. Fenestration can serve as a physical and/or visual connection to the outdoors@ as well as a means to admit solar radiation. The solar radiation provides natural lighting@ referred to as daylighting@ and heat gain to a space. Fenestration can be fixed or operable; operable units can allow natural ventilation to a space and egress in low-rise buildings.