This chronometric technique is the most precise dating tool available to archaeologists who work in areas where trees are particularly responsive to annual variations in precipitation, such as the American Southwest.
Developed by astronomer A. E. Douglass in the 1920s, dendrochronology—or tree-ring dating—involves matching the pattern of tree rings in archaeological wood samples to the pattern of tree rings in a sequence of overlapping samples extending back thousands of years. These cross-dated sequences, called chronologies, vary from one part of the world to the next. In the American Southwest, the unbroken sequence extends back to 322 B.C.
So, when an archaeologist finds a well-preserved piece of wood—say, a roof beam from an ancient pithouse—dendrochronologists prepare a cross section and then match the annual growth rings of the specimen to those in the already-established chronology to determine the year the tree was cut down.
Read how A. E. Douglass pioneered the science of tree rings in this 1929 National Geographic article titled "The Secret of the Southwest Solved by Talkative Tree Rings." Includes numerous fascinating historic photographs. (Article available on the Indiana State University website.)
The Laboratory of Tree-Ring Research in Tucson is the world's oldest dendrochronology lab; their website includes information for researchers and the general public.
The Science of Tree Rings is an educational website with lots of information—from basic definitions and principles to links to tree-ring databases and other resources.