Al-Khazneh (the Treasury) in Petra, capital of the Nabataean Kingdom, reverses architecture's usual logic. Most buildings rise by placing material on top of material — by addition. The Treasury, by contrast, is revealed by removing material from the sandstone rock — by subtraction.

The facade, some 40 metres tall, is carved directly into a sheer pink-sandstone cliff at the end of a narrow gorge (the Siq). Structurally there is no load-bearing member or arch here; the columns, pediments, and niches are not decorative but volumes "left behind" in the mass — they carry no load because the rock behind them already carries everything. This is the vocabulary of Hellenistic architecture (Corinthian columns, broken pediment, tholos) transferred to the cliff as a facade — but not built, carved.

The work proceeds from the top down: the masons began at the uppermost cornice and worked downward, so no scaffolding was needed and the finished upper parts were left untouched. The interior behind the facade is likewise a set of plain rooms hollowed by the same process. Petra thus shows a rare face of architecture: form obtained not by addition but by cutting away stone — like a sculpture — with structure, facade, and interior volume emerging from one and the same mass.