First, contrary to the single ring per year paradigm, alternating poor and favorable conditions, such as mid-summer droughts, can result in several rings forming in a given year.
These patterns can be compared and matched ring for ring with trees growing at the same time in the same geographical zone (and therefore under similar climatic conditions).
When these tree-ring patterns are carried back, from tree to tree in the same locale, in overlapping fashion, chronologies can be built up—both for entire geographical regions and sub-regions.
European chronologies derived from wooden structures initially found it difficult to bridge the gap in the 14th century when there was a building hiatus, which coincided with the Black Death, Given a sample of wood, the variation of the tree-ring growths provides not only a match by year, it can also match location because the climate across a continent is not consistent.
This makes it possible to determine the source of ships as well as smaller artifacts made from wood but which were transported long distances, such as panels for paintings and ship timbers.
The techniques of dendrochronology are more consistent in areas where trees grew in marginal conditions such as aridity or semi-aridity where the ring growth is more sensitive to the environment, rather than in humid areas where tree-ring growth is more uniform (complacent).
In addition, some genera of trees are more suitable than others for this type of analysis.
Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed in order to analyze atmospheric conditions during different periods in history.
Dendrochronology is useful for determining the timing of events and rates of change in the environment (most prominently climate) and also in works of art and architecture, such as old panel paintings on wood, buildings, etc.
Douglass sought to better understand cycles of sunspot activity and reasoned that changes in solar activity would affect climate patterns on earth, which would subsequently be recorded by tree-ring growth patterns (i.e., sunspots → climate → tree rings).
Diagram of secondary growth in a tree showing idealised vertical and horizontal sections.
Moreover, wood from ancient structures with known chronologies can be matched to the tree ring data (a technique called cross-dating), and the age of the wood can thereby be determined precisely.