You catch the swap.

Number 8 trying to live as 9, or a Gareth-curve that almost fooled a tired eye. The tracing floor tells the sky the truth. The stone comes out. The right stone goes in. An hour walks away. A year of crack does not arrive.

Rowan says, “The floor is the master when I am on the road.” It is the best sentence you have ever been lent.

The vault, when it locks, locks without a lie in its ribs. Iseult promises a window that will not sit over a future split. Peverel notes voussoirs checked as if that were a kind of silver.

You eat. The fossil comma in the keystone is still not a saint. It does not need to be.

The swapped voussoir sits on the yard earth, shame-faced, still good stone. It will go where its number lives. You eat with lime on your bread and do not mind. The tracing house door stands open: a pale field that just saved a choir.

THE END

What you learned: voussoirs are numbered wedge-stones; a swapped one is a crack waiting. The tracing floor is how you catch a lie before the vault hides it.