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Centre:
Definition:

Temporary timber frame, set in an opening, on which an arch is turned; used for larger spans say over 1.5m. Two sides are framed to the curve of the arch and joined by laggings to make a centre the thickness of the arch soffit. The centre is positioned on Folding Wedges, placed on the supporting struts, so that it may be levelled and later eased when the arch is complete and set.