1) Often used in the plural, timber framework which supported a stone arch until all the keystones were safely in place.
The OED has examples of this word from 1611 but it actually has a much longer and more complicated history than that: 1675 wood for centres, Ilkley Bridge. It was not a term confined to bridge-building nor even to arches, and was used more generally for substantial roof timbers in barns and houses. When Beverley Bar was being constructed, in 1409, the accounts included a charge of 6d pro cariagio de syntyrs, and later, in the same document, two carpenters were paid for making fenestras et syntyrs: these must have been ‘centres’ although they were not necessarily supporting an arch. In 1703, the disbursements for Pickering tithe barn included one paire of senters valued at 15s and a more explicit reference in 1754, in Hutton-le-Hole, was to 2 pair of centres or Furks standing towards the East end of the barn. These pairs of ‘centres’ may have been crucks that supported the ridge timbers. An early use of the word in connection with bridges is in the Lady’s Bridge agreement: 1485-6 the making of centres shall be of both theyr costys, Sheffield, and an unusual spelling occurs in the Catterick Bridge contract: 1422 the landstathes and the seentrees. Such spellings are an indication that some clerks believed that ‘tree’ was actually part of the word and they almost certainly explain the previously obscure term ‘signtree’, which is commonly found in other building records in Lancashire and Yorkshire. This is dealt with separately under ‘seentree’.