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When making an opening, you have the classic lintel, the more efficient arch, and right there in the middle, the corbeled arch.

A lintel is one piece that lays flat, suspended above the ground by two others. Stonehenge is an example of this.

But that will only give you openings as wide as your lintel. When you're building with stone, that's probably not very long. You can get bigger openings with an arch, with the added advantage of using smaller rocks. The defining characteristic of an arch is the keystone, which the other voussoirs lean on. The weight is directed sideways, not down.

But before there were arches there were corbeled arches. These are not arches, but they do allow you to have an opening bigger than you lintel.

To make a corbeled arch you offset blocks so as to make a case of inverted 'steps' over the opening. The first block might extend 12cm past the door frame. The next one extends 12cm father than that one. Soon the bocks on one side of the doorway are nearing those of the other side, and you can top it off with a normal sized block as the lintel.

There is no keystone. It can fall inwards easily, and most of its strength will come from the wall it is set in, holding it in place.

The best way to envision this is probably with legos. Try making an arch out of them. Unless you've done something tricky, that's a corbeled arch.

A simple drawing would make this WU largely redundant. It's a simple idea, really.

            |____|     - the lintel
          |__|  |__| 
         |__|    |__| 
        |__|      |__| 
       |__|        |__| 
      |__|          |__|
      |__|          |__|
     |___|          |___|
A good example of corballing (as I would have spelled it) can be found in the beehive huts on the Skelligs in Ireland. These hemispherical structures were created by laying rings of stone on top of each other, each ring being smaller than the last. The cusp of the curve is then capped with a massive keystone. The weight of this stone is distributed throughout the walls of the structure, allowing the whole thing to stand up for over a thousand years, without the aid of any kind of mortar.

Ironically, all of the original beehive huts, dating from the ninth century, are still intact on Skellig Michael, while an oratory built in a more familiar rectangular shape, with the use of mortar, lies in ruins, despite being built several hundred years later.

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