A planar truss is one where all members and nodes lie within a two-dimensional plane, while a space frame has members and nodes that extend into three dimensions. The top beams in a truss are called 'top chords' and are typically in compression, the bottom beams are called 'bottom chords', and are typically in tension. The interior beams are called webs, and the areas inside the webs are called panels, or from graphic statics (see Cremona diagram) 'polygons'.