Dendrochronology, from the Greek dendron (tree) and chronology (time) is the process of dating historic timbers through the scientific study and comparison of tree rings to establish a felling date or date range. 

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The way dendrochronology works is relatively simple. As a tree grows, it puts on a new growth or tree-ring every year, just under the bark. Trees grow, and put on tree-rings, at different rates according to the weather in any given year: a wider ring in a favourable year and a narrower ring in an unfavourable year. Thus, over a long period of time (say 60 years or more) there will be a corresponding sequence of tree-rings giving a pattern of wider and narrower rings which reflect droughts, cold summers, etc. In effect, the span of years during which a tree has lived will be represented by a unique fingerprint, which can be detected in other geographically-similar tree-ring chronologies.

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After taking core samples from construction or archaeological timber, the samples are carefully prepared and measured. Using the natural variation in growth pattern described above, dendrochronologists are able to determine when the tree was growing and when it was felled. As we know green oak was used almost immediately or stockpiles for only a short period, we can often provide dates to the season and year of felling and likely construction, if we have that last year of growth surviving. 

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Ross Cook works and an Associate Dendrochronologist of the Oxford Dendrochronological Laboratory, with Dr Martin Bridge and Dr Dan Miles with whom all dendrochronology is undertaken. Website: www.oxford-dendrolab.com