Antoni Gaudí's Casa Milà (popularly La Pedrera, 'the quarry') realises one of modern architecture's central ideas — that the wall need no longer carry the building — with traditional stone, in an unexpected place.

Until then the outer wall of a stone building was also load-bearing: thick, hard to pierce, with limited windows. At Casa Milà Gaudí lets an inner structural system — stone and iron columns and beams — take the load; the undulating cut-stone façade outside is a self-carrying 'curtain' hung from that frame. Freed from any structural duty, Gaudí shapes the façade freely: the solid stone curves continuously like a sea cliff or a wave; windows open wherever and at whatever size is wanted. This is an early, organic herald of the principle Le Corbusier would theorise twenty years later as the 'free façade' (façade libre).

Inside there are no load-bearing walls either; thanks to two central courtyards and surrounding columns each floor has a free plan whose partitions can be arranged at will. The attic is carried by a row of parabolic brick arches — another instance of Gaudí's devotion to catenary geometry. Casa Milà turns an ordinary programme, an apartment block, into a manifesto of separating structure from envelope; it is a precursor of the modern free plan and free façade, written in stone before the steel-and-glass aesthetic.