When applying this principle to dendrochronology, it says that the variations in conditions present today must have been present in the past.This does not mean to say that the conditions are exactly the same but that similar kinds of influences affected the similar kinds of processes.Samples taken from trees of unknown age can then be studied for matches with samples from trees with known sequences of growth.Using this process, when the rings "match" or are found to be overlapping in age, we are able to "see" even further back in time.
Its outer growth rings were compared with the inner rings of a living tree.
These must be compared not only with samples from other trees at the same location but also with those at other sites in the region.
Additionally, the average of all data provides the best estimate of climate averages.
Further checking is done until no inconsistency appears.
Often several sample cores are taken from each tree examined.