Radiocarbon dating after 1950 identifying dating bottles
See also ORAU's Explanation of Radiocarbon Results.
Radiocarbon dates should always be reported either as `percent modern' or years `before present' (BP).
The first indicates the proportion of radiocarbon atoms in the sample as compared to samples modern in 1950.
The second is directly derived from this on the assumption that the half-life of radiocarbon is 5568 years and the amount of radiocarbon in the atmosphere has been constant.
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.
Once calibrated a radiocarbon date should be expressed in terms of cal BC, cal AD or cal BP.
The cal prefix indicates that the dates are the result of radiocarbon calibration using tree ring data.
For two important reasons, this does not mean that the sample comes from 3619 BC: Many types of tree reliably lay down one tree ring every year.
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.
This is very useful as a record of the radiocarbon concentration in the past.