and ancient wood
and counting the annual growth rings for, usually, archaeological
purposes. But hey, some people might do it just for kicks
Mostly, it is used for the dating of past events such as climate change. Larger rings mean more moist seasons, where a tree diverts energy to growth, smaller rings show harsher times.
In this way, dendrochronology can be used to trace rainfall, wind, snow, droughts, fires and volcanic activities throughout the life of the tree. The more varied the growth rings, the more a trees growth has been limited by changing environmental conditions. Similiary, the more constant the growth rings, the more a tree has led a fairly easy life, and climate conditions have been stable.