The deeper something is found, the older it generally is. Volcanic activity, earthquakes and the unpredictability of rock formation and accumulation can make the chronology established by this method inaccurate.
Bone can also be dated through fluorine analysis, another popular relative dating technique.
Without relative and radiometric dating techniques, no one would know whether dinosaurs lived 5,000 or 150,000,000 years ago.
Comparing the fluorine content of two or more fossilized bones found at the same site can therefore show which are older and which are more recent.
Using established knowledge of potassium-40's half-life, which is the rate at which it decays and changes into argon-40, scientists can date rock material that is millions of years old.
In fact, K/Ar dating has been used to date the age of the earth itself.
This happens at predictable rates, which makes radiometric dating a more accurate and reliable indicator of age than relative dating.
One of the major radiometric dating techniques used today measures the decay of potassium-40.