Dendrochronology is the science of tree-ring analysis. It derives from the Greek root words dendron for tree and chronos for time. Thus, dendrochronology uses the history of tree growth, as expressed in its annually produced radial growth rings, to provide information about the past. The realization that trees growing in suitable environments put on one annual ring of growth per year has a very long history going back at least to Theophrastus in ancient Greece. However, it was not until the early twentieth century that the basic principles of dendrochronology were formally developed and exploited. Annual tree-ring chronologies can also be used to reconstruct past climate. This branch of dendrochronology is called dendroclimatology.