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85 Penn

We want to build the arch as a cantilever rather than the traditional way of putting in centring. That means we have to stop the blocks falling off till the keystone goes in. We need something to hang each voussoir on and something to hold it back. String from the top of the handrails does the latter.

The handrail post seen here is on the upper voussoir. It overhangs slightly and so forms a lip which fits over the edge of the lower one. By fixing the step riser to the back edge of the post it hangs well back over the lower voussoir and so makes a locator to stop slipping.