1 Segmental arch with wooden former (centring) in place

2 Round (Roman) arch with centring in place and standing on two jacks which can then be lowered to release it when the mortar is set.

3 Three-centred arch. Here the word centre means a point where the point of the compass is set to draw the arc.

4 Round wooden former for a Roman arch

5 Another type of four-centred arch.

6 A Florentine arch. There is a row of these above a terrace of shops in Essex Road. They were made popular in England when Ruskin published his book The Stones of Venice, 1851-53. From then Florentine arches became popular and a number can be found in Stoke Newington.
These diagrams show several types of arch and
some of the formers used to build the arches.