In a very dry year growth will be restricted, and the ring narrow, while a wet and humid year will produce luxuriant growth and a thick ring.
By comparing a complete series of rings from a tree of known date (for example, one still alive) with a series from an earlier, dead tree overlapping in age, ring patterns from the central layers of the recent tree and the outer of the old may show a correlation which allows the dating, in calendar years, of the older tree.
It is then compared to a master chronology in dendrochronology studies.
The central rings of this older tree may then be compared with the outer rings or a yet older tree, and so on until the dates reach back into prehistory.
Problems that arise are when climatic variation and suitable trees (sensitive trees react to climatic changes, complacent trees do not) are not be present to produce any significant and recognizable pattern of variation in the rings.
Since timber can be dated by radiocarbon, dates may be obtained from dendrochronologically dated trees.
It has been shown that the radiocarbon dates diverge increasingly from calendrical dates provided by tree-rings the further back into prehistory they go, the radiocarbon dates being younger than the tree-ring dates. Douglass first showed how this method could be used to date archaeological material.