Dendrochronology is a method of dating timber by comparing its pattern of tree-rings with a reference tree-ring chronology.
Every year a living tree puts on a new layer of growth under the bark. In years when conditions are favourable to growth, the width of the layer – the tree-ring – will be greater than when conditions are not as favourable. Trees living at the same time, in the same area, will exhibit similar patterns of growth. A reference tree-ring chronology can be constructed by overlapping the ring patterns of successively older timbers from trees of the same species – in Britain, oak, the most commonly used building-timber, is the species used in dendrochronology – starting with living trees and working backwards through historical samples. A sample of the timber to be dated is matched against the reference chronology – the sample must have more than fifty rings for a reliable match – which will provide a date for each ring. If a bark edge is present in the sample, then the date of the outer ring will be the precise year the tree was felled – it may even be possible to judge the time of year the felling took place. If the bark has been trimmed off, then the tree was alive during year indicated by the final ring, and must have been felled some time afterwards.