If you’ve ever seen a picture of Stonehenge, the prehistoric monument near Salisbury, England, you’ll recognize the structural system known as post and lintel. It consists of two upright blocks — the posts — with another block — the lintel — spanning the gap between them. This was the earliest type of structure used to span open space.

The trouble was, the widest distance you could span with a post and lintel was limited to the size of the biggest stone lintel you could get your hands on — not to mention lift into place. Hence, even the grandest ancient buildings under roof — Egypt’s vast temple at Karnak, for instance — were little more than forests of stone columns with narrow passages left over in between.

The invention that finally overcame this problem was the arch. It had humble enough beginnings. It was already known to the Babylonians, Assyrians, Greeks and others, often in the form of a roof over underground drains. Yet the Romans were the first to really exploit its unique structural properties. They recognized that, unlike a lintel, an arch could span distances much greater than any single block in the structure.

Here’s why: A stone lintel carries a load by bending infinitesimally, which compresses the upper half and stretches the bottom half. Now, stone is strong under compression but fairly weak under tension, so the lintel has to be really deep relative to its span or it may crack.

An arch works differently. Because the wedge-like stones are arranged in a circular shape, placing a load on top squeezes the blocks against each other, compressing them but not putting them under tension. This takes much better advantage of the strength of the stone, and hence requires a lot less of it to do the job.

Of course, an arch can’t just stand up by itself — something has to brace it on either side to keep it from spreading apart under load. Usually, a bit of solid wall serves this purpose. But you can also put a series of arches side by side to form a continuous arcade, letting one arch brace the next, as in the elevated Roman aqueduct known as the Pont du Gard near Nimes, France.

Arches form the basis of even more useful structural systems, though. By stretching an arch into a tunnel shape you get a vault, an early means of roofing over space without needing intervening supports. By intersecting two vaults at right angles you get a groin vault, which the Romans used to roof their monumental Baths of Caracalla, among other things. And if you rotate an arch about its centerline, you get a dome — the basis of some of man’s greatest architectural works, from Rome’s Pantheon to Istanbul’s Hagia Sophia.

Ironically, since modern structures are now seldom built of masonry blocks, true arches are rarely used anymore except for dramatic effect or, occasionally, to support bridges, trestles, and the like. What’s most commonly used instead? Why, it’s our prehistoric pal, the post and lintel — though nowadays it’s made of wood, steel or concrete.

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