In 1958 Hessel de Vries showed that the concentration of carbon-14 in the atmosphere varies with time and locality. The relatively short-lived C is constantly renewed by cosmic ray bombardment on atmospheric nitrogen. Since the bombardment is slightly variable, and for other reasons, the C taken into organic matter is also slightly variable. That leads to errors in the chronology. However, under about 20,000 years the results can be compared with dendrochronology, based on tree rings. For the most accurate work, variations are compensated by means of calibration curves. 