Dating method
Dendrochronology
Dating wood from the pattern of its rings. It dates the growth of the tree, and only a last ring under the bark dates the felling.
What is measured
Trees of the same region share wide and narrow rings as rainfall and temperature change. A timber’s ring pattern can be slid along a master sequence until it fits, which gives calendar years for those rings.
If the outer ring and the bark are missing, the date is a year the tree was still alive, not the year it was cut. Builders also reused old beams. A ring date on a beam can be older than the wall it was found in.
What it does not do alone
Charcoal that has lost its outer rings, or a pattern that fits more than one place on the master sequence, does not pick a single building year. A regional master sequence has to exist. Without it, the rings are a relative pattern, not a calendar.
A dendrochronological date is strong when several timbers from one phase end in the same year and the bark is there. One reused beam is a warning, not a founding date.