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The growth of trees is shown by the annual wood growth, under the bark, ring the width of which is a function of the rainfall and the period of sunshine during the vegetative period, the spring and the summer: few wood and narrow ring dry years, a lot of wood and wide ring wet years.
Between extremes all the variations are possible. The trees of the same region being subjected to the same meteorological regime, it is possible to track down on every trunk the sequences of identical variations.
So to correlate among them and to know so the date of their slaughter.

Absolute Dendrochronology

The principle of the dendrochronology is based on the comparison of the thickness rings of growth: for a good reliability she should be applied in the biggest possible number of trees.
On the basis of thousand samples, it is then possible to build a reference curve, "a standard" curve which characterizes the annual variations of humidity and heat for one given geo-climatic sector and this during the longest possible duration. In this standard curve is compared quite other curve established on a trunk.
To study, to look for concordances, so chronological place: it is the absolute dendrochronology. All these measures are treated by computer what accelerates and refines the results and the specialists established today a scale of time until seven millenniums B.C. with the oak tree.

Relative Dendrochronology

The principle is identical but here, the study concerns only a lot of trunks from the same site; making the comparison between them we know the time which separates the dates of cutting down.
It is what allowed in Charavines to establish the sequences of the villages architecture.