Absolute dating (or radiometry), based on the principle of disintegration of certain radioactive elements, establishes the age of fossils. Carbon 14 (C14) dating is the best known, as carbon is an element found in every living organism. Because organisms contain C14 and C12 in a proportion that is stable and C14 begins to disintegrate at a steady rate when organisms die, the amount of time since a plant or animal has died is measured by establishing the proportion of C14 remaining in relation to the C12.