Because rainfall patterns vary annually, any given set of tree ring patterns in a region will form a relatively distinct pattern, identifiable with a particular set of years.
All of these techniques have proven somewhat unreliable.
Museums sometimes use them to determine if a ceramic is an antique or a modern forgery.
These techniques are based upon the measurement of radioactive processes (radiocarbon; potassium-argon, uranium-lead, uranium-thorium, thorium-lead, etc.; fission track; thermoluminescence; optically stimulated luminescence; and electron-spin resonance), chemical processes (amino-acid racemization and obsidian hydration), and the magnetic properties of igneous material, baked clay, and sedimentary deposits (paleomagnetism).
Other techniques are occasionally useful, for example, historical or iconographic references to datable astronomical events such as solar eclipses (archaeoastronomy).
Absolute dating can be achieved through the use of historical records and through the analysis of biological and geological patterns resulting from annual climatic variations, such as tree rings (dendrochronology) and varve analysis.