Radiocarbon dating and tree-dating, in combination, have provided a very powerful tool to establish a time spectrum for more recent dates in the past. The initial idea for dating by tree rings can be traced back to 1811. Modern scientific tree-ring dating, dendrochronology, stems from pioneering work in early 1900s. Usually, but not always, trees produce one ring each year. This ring is formed by the cambium, which lies between the old wood and the bark. In spring, wood cells with large lumens are manufactured, but in summer and autumn, the cells become smaller and more thick-walled until the onset of winter, in which the production of a new cell stops. The same process is repeated in the following year. In this way a year’s growth (annual ring) is imprinted as new wood. The demarcation line between summer and autumn wood of the previous year, with its characteristic small cells, and the spring wood of the year following, with its large cells, enables annual rings to be counted relatively easily. Growth rings, however, are not always the same thickness. They vary for several reasons. Environmental factors rigidly control the degree of growth of an annual ring or determine whether, in fact, an annual ring appears or not in any particular year. Thus in a specific locale or, more accurately, a specific climatic province, tree-ring counts will reflect climatic conditions and variations due to inequalities of climate from year to year. In years with abnormal drought, for example, narrow rings are produced and sometimes no ring at all. In this way a fossil record is imprinted for as long as the wood remains intact. From this pattern a historical template can be constructed to correlate one set of growth rings in one tree with a set of growth rings in another tree or piece of timber. Another important factor is that tree-ring growth varies with age of the tree. As the tree matures, the rings become narrower, and this results in the central rings being wider than those on the outer part of the tree.