Corbelling (the false arch and false dome)

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Spanning an opening by stepping successive courses of masonry inwards until they meet, then usually trimming the profile smooth. Each course is held down by the weight of the wall behind it, so the stones are cantilevers, not voussoirs: nothing is wedged, and the structure works by bending and self-weight rather than by pure compression along a curve. Corbelling is the older technique and it is spectacular within its limits - the Grand Gallery of the Great Pyramid and the Mycenaean tholos tombs are both corbelled - but it needs an enormous mass of masonry above and behind the opening, so it becomes uneconomic quickly as the span grows. A corbelled profile looks like an arch and is not one, and the distinction is the single most useful thing a visitor can learn to see.