service spaces between the slabs and the ceilings the curtain wall is mounted on the outer edges of the load bearing structure by angle form fixing cleats 12 to which vertical mullion members 14 of the structure are secured by bolts 16 horizontal transom members 18 extend between laterally adjacent mullion members to define with them a series of rectangular infill spaces typically the transom members 18 are located at the levels of the suspended ceilings c and of sill members s on the top of the upstands so that between a ceiling and the sill above it wall panels w are used as infill while a glazing infill g is used between the sills and the ceilings above them the mullion members 14 are shown terminating at the sill transoms vertically spaced ends of vertically adjacent mullion members facing each other whereby an expansion gap 20 is provided between the adjacent mullion members but it will be understood that other arrangements are possible it only being required that each mullion member should be secured to one floor slab so as to transmit to that slab the gravity load on it and that the ends of the mullion member should coincide with transom members of the curtain wall structure the mullion members are extruded metal hollow box sections and adjoining members are interconnected by elongate spigots 22 that are a close sliding fit in the members at one end each spigot is fixed relative.