The voussoir was a wedge-shaped stone in an arch. Each stone in the arch was a voussoir. The voussoir was wider at the top than at the bottom and when you stacked them in a curve each wedge pushed against the wedge next to it and the pushing held the arch up. The arch held itself up by the pressure of each stone trying to fall and failing. The voussoir was the stone that could not fall because the other stones would not let it. The arch was a community of wedges.

The keystone was the voussoir at the top. The last stone placed. The stone that locked the arch and turned a pile of wedges into a structure. Before the keystone went in the arch was scaffolding. After the keystone went in the scaffolding came out and the arch stood on its own. The keystone was the moment the arch stopped needing help. Every project has a keystone. The piece that goes in last and makes everything before it permanent. You do not know which piece is the keystone until the scaffolding comes down.

The voussoir transferred the load. The weight above the arch pushed down on the keystone and the keystone pushed sideways on the voussoirs next to it and the voussoirs pushed sideways on the next voussoirs and the force traveled down the curve until it reached the springer at the bottom where the arch met the wall. The wall absorbed the sideways force. The arch turned a vertical load into a horizontal thrust. The arch was a translator. The arch spoke the language of the wall and the language of the sky at the same time.

The Romans built voussoir arches that are still standing. Two thousand years of rain and frost and war and the voussoirs have not moved because the geometry has not changed. The wedge angle is the same today as it was when the mason cut it. The geometry does not decay. The stone decays. The mortar decays. The geometry is immortal. The arch is the proof that mathematics outlives material. The voussoir will crumble eventually but the angle it was cut to will be true forever.

Nobody cuts voussoirs by hand anymore. The concrete arch is poured into a form and the form makes the curve and the curve has no joints. The poured arch is one piece. The voussoir arch was many pieces. The many pieces could be repaired one at a time. Pull a damaged voussoir and slide a new one in and the arch does not notice. The poured arch cannot be repaired one piece at a time because there are no pieces. Crack the concrete and you crack the whole arch. The voussoir arch was a sentence made of words. The poured arch is a sentence made of one long syllable that cannot be edited.

See also: Spandrel, Ashlar