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Visible rings result from the change in growth speed through the seasons of the year, thus, one ring usually marks the passage of one year in the life of the tree.
The rings are more visible in temperate zones, where the seasons differ more markedly.
Thus wood from ancient structures can be matched to known chronologies (a technique called cross-dating) and the age of the wood determined precisely.
Cross-dating was originally done by visual inspection, until computers were harnessed to do the statistical matching.
Adequate moisture and a long growing season result in a wide ring. Alternating poor and favorable conditions, such as mid summer droughts, can result in several rings forming in a given year.
Timber core samples measure the width of annual growth rings.
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Likewise, in areas where trees grew in marginal conditions such as aridity or semi-aridity, the techniques of dendrochronology are more consistent than in humid areas.
These tools have been important in archaeological dating of timbers of the cliff dwellings of Native Americans in the arid Southwest.