Page "Radiometric dating" Paragraph 27

One of its great advantages is that any sample provides two clocks, one based on uranium-235's decay to lead-207 with a half-life of about 700 million years, and one based on uranium-238's decay to lead-206 with a half-life of about 4. 5 billion years, providing a built-in crosscheck that allows accurate determination of the age of the sample even if some of the lead has been lost.

This can be seen in the concordia diagram, where the samples plot along an errorchron ( straight line ) which intersects the concordia curve at the age of the sample.