Centring

Centring[1], centre[2], centering[3][4], or center[5] is a type of falsework: the temporary structure upon which the stones of an arch or vault are laid during construction.

Until the keystone is inserted an arch has no strength and needs the centring to keep the voussoirs in their correct relative positions.

[3] A cross piece connecting centring frames is called a lag or bolst.

[6] Centring is normally made of wood timbers, a relatively straightforward structure in a simple arch or vault; but with more complex shapes involving double curvature, such as a dome or the bottle-shaped flue in a Norman-period kitchen, clay or sand bound by a weak lime mortar would be used.

Shaping could be done by eye, perhaps with the help of a template, then stones or bricks laid against it.