Most trees produce a ring of new wood each year and these circles of growth can easily be seen in a cross-section of the trunk of a felled tree. These rings are not of uniform thickness. In an individual tree, they will vary for two reasons. First, the rings become narrower with the increasing age of the tree. Second, the amount a tree grows each year is affected by fluctuations in climate.(...) Dendrochronologists measure and plot these rings and produce a diagram indicating the thickness of successive rings in an individual tree. Trees of the same species growing in the same area will generally show the same pattern of rings so that the growth sequence can be matched between successively older timbers to build up a chronology for an area. (...) By matching sequences of rings from living trees of different ages as well as from old timber, dendrochronologists can produce a long, continuous sequence extending back hundreds, even thousands, of years from the present.