These are the basis for the calibrations performed by the programs like CALIB and Ox Cal. Calibration of radiocarbon determinations is in principle very simple.
If you have a radiocarbon measurement on a sample, you can try to find a tree ring with the same proportion of radiocarbon.
If we have a tree that is 500 years old we can measure the radiocarbon in the 500 rings and see what radiocarbon concentration corresponds to each calendar year.
Since the calendar age of the tree rings is known, this then tells you the age of your sample.
In practice this is complicated by two factors: These effects are most clearly seen by looking at a specific example.
See also ORAU's Explanation of Radiocarbon Results.
Radiocarbon dates should always be reported either as `percent modern' or years `before present' (BP).
The results of calibration are often given as an age range.
In this case, we might say that we could be 95% sure that the sample comes from between 1375 cal BC and 1129 cal BC.To extend this method further we must use the fact that tree ring widths vary from year to year with changing weather patterns.By using these widths, it is possible to compare the tree rings in a dead tree to those in a tree that is still growing in the same region.Radiocarbon measurements are always reported in terms of years `before present' (BP).This figure is directly based on the proportion of radiocarbon found in the sample.The wood in these rings once laid down remains unchanged during the life of the tree.