The method relies upon the response of trees to the weather during the growing season, which runs from March to October.
(It should be noted that there is no direct linear relationship between ring width and, say, sunshine, or other weather components) Thus a 'good' or 'poor' growing season is defined with reference to the amount of growth produced.
For example, the year 1976 had a gloriously hot, long summer with most rainfall arriving in autumn but the trees did not appreciate it and all oaks produced a distinctively narrow ring.
Because of local, non-climatic causes of change of growth width, the chronologies around the country vary somewhat, and the best dating match is always obtained from a local regional master chronology.
The dendro-date is thus the year in which the final ring of the specimen grew (the year in which the tree was felled, but not necessarily the year in which the building was constructed).
Almost certainly the century or portion of a century when it was built may be assigned with some certainty.
However, as more and more work is done and increasing numbers of structures with complex constructional phases are encountered, the general features may not be sufficient to give the accuracy in dating that is currently required.
Again the summer of 1915 was cold and wet, quite different from 1976, yet the trees also produced a distinctly narrow growth ring.
So it will be seen that seasons that are hot and dry as well as those that are cold and wet will produce a narrow ring so that such a ring is not diagnostic of the weather.
The youngest patterns are obtained from living trees, where the felling date of the final ring is known.
Progressively older patterns are obtained from trees in recent buildings, older buildings, archaeological sites and ancient bog oaks.
In order to obtain an accurate match and hence a date, it is important to have at least 80 rings on the specimen that is to he dated.