Structural forms ,such as the beam or the arch, have developed through the ages in relation to the availability of materials and the technology of the time ,.The arch ,for instance ,undoubtedly developed as a result of the availability of brick .In the technology of buildings ,every structure must work against gravity ,which tends to pull everything down to the ground .A balance must therefore be attained between the force of gravity ,the shape of the structure ,and the strength of the material used .To provide a cover over a sheltered space and permit openings in the walls that surround It ,builders have developed four techniques consistent with this balance between gravity, form ,and material .These building techniques are post and lintel ,arch and vault ,truss ,and cantilever construction.