English: Principles of dating samples with an isochron. The concentration of a radioactive mother isotope (N) is plotted against the concentration of a radiogenic daughter isotope (D*), both normalized by dividing by the concentration of a non-radiogenic stable isotope of the daughter element (D_ref). Different phases or components in the same system are measured (for example: different minerals in the same rock). Individual phases are assumed to have been isolated since their formation: mother and daughter were unable to escape. The system is assumed to have had a homogeneous isotopic composition at the start. This means that the initial ratio of the daughter with the non-radiogenic isotope of the same element (D₀) was the same for all phases. Both values for mother and daughter are normalized by dividing over the abundance of this non-radiogenic isotope of the daughter element. The figure shows the evolution of three phases with different initial amounts of the mother (in orange, pink, and blue). The crystals show the relative abundances of mother isotopes (in red) and daughters (in green) for the plot points in the graph. As time goes by, the phases always plot on a line (the isochron), but the slope of this line increases with time. Thus, the age of the system follows from the slope of the isochron (namely: slope = e^λt-1 in which λ is the half-life of the mother isotope).