Isotopes of a particular element have the same number of protons in their nucleus, but different numbers of neutrons.

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Radiocarbon dates are presented in two ways because of this complication.

The uncalibrated date is given with the unit BP (radiocarbon years before 1950).

From these records a “calibration curve” can be built (see figure 2, below).

A huge amount of work is currently underway to extend and improve the calibration curve.

In this way large domed tombs (known as tholos or beehive tombs) in Greece were thought to predate similar structures in the Scottish Island of Maeshowe.

This supported the idea that the classical worlds of Greece and Rome were at the centre of all innovations.

The calibrated date is also presented, either in BC or AD or with the unit cal BP (calibrated before present - before 1950).

The calibrated date is our “best estimate” of the sample’s actual age, but we need to be able to return to old dates and recalibrate them because new research is continually used to update the calibration curve.

Some of the first radiocarbon dates produced showed that the Scottish tombs were thousands of years older than those in Greece.