Each year a tree lives, a new ring of wood appears in its trunk. These rings can be counted, and the total number reveals the tree's age. Each of these rings vary in size, and this variation is related to the climate. A thick ring indicates favorable growing conditions, meaning plenty of water and sun, and mild weather. A thinner ring suggests harder times, perhaps drought, a cold summer, or a damaging fire. This pattern of thick and thin rings is shared by all the trees growing in a particular region at the same time.
Cross-dating: This means finding matching ring patterns from several samples in the same region. A match means you can identify the exact year in which each ring was formed. Such matches have allowed historians to pinpoint the year, and often the season, in which timber was cut.
Core sampling involves extracting a thin core from the trunk without damaging the tree. Scientists usually extract a 5mm core of wood with a "increment borer," a special instrument with a hollow shaft that is screwed into the trunk of a tree. A long spoon called an extractor is then inserted into the shaft, and the core sample is withdrawn from the tree. Core samples can also be taken from fallen timber, fence posts, and logs used in old construction.
Once a core has been collected, reading it can be tricky. First the cores are sanded to bring out the sample's detailed anatomy. Then they are measured, and a calendar year is assigned to each ring. This must be done carefully by hand.
To protect against errors, additional core samples are usually gathered from different trees at a site. After the cores have been analyzed, the average size of the rings is computed for each year. This information is then used to compile a dendrochronology for the region.
Computer software compares core samples from old trees to core samples taken from historic structure. The software can determine where the wood from the historic structure fits among the typically older tree sample. For example the wood from the tree may have rings from 1706 to 2006. The tree ring pattern for the historic structure might match the pattern from 1755 to 1825. Thus we know the structure was built with wood that was cut in 1825.