Part of what’s interesing to me is (1) they know how old the wood was when felled (that’s the easy part, count the rings) (2) they know when it was felled (Line up the rings with growth pattern records to lock down the time it grew) and, to me the new part, (3) where it was felled. How do they know which forest it came from? Interesting.
It requires a chronograph study of forests in the vicinity, generally done with a microscope.
Dendrochronology also includes a study of surrounding forests and climate, and doing a comparison.
If the source forest was hundreds of miles away, you generally depend on someone else's ring study.
By climate study,I means that it includes temperature, humidity, rainfall, and knowledge of terrain and topography,
as well as the location of where the rings were found.
Basically, it is a scientific system of comparative study and analysis of known factors within a geographic area.
It is a field of study within archeology and anthropology.