Dendrochronology is also called tree-ring dating. It is a way of dating a piece of wood, based on the size and pattern of its rings. It works because living trees add a ‘growth ring’ to their trunk and branches every year.
The size of the rings depends on the conditions where the tree is growing. In dry years the ring is thin, whereas in wetter years the ring will be thicker. Trees of the same type growing in the same region will have similar patterns of tree-ring growth. Experts have created master tree-ring profiles for different regions showing the pattern of thin and thick rings (a bit like a barcode!). When pieces of timber of an unknown date are found, they can be compared to the master profile. By matching up the order of thin and thicker rings, the piece of timber can be dated!
A cross-section showing annual rings, in a type of pine tree called Pinus taeda (or loblolly pine)
Image attribution: "Pinus taeda crossx7358" by Pollinator at the English language Wikipedia. Licensed under CC BY-SA 3.0 via Commons