An isochron refers to a curve or a surface that describes the state of multiple systems with the same age. In geochronology, an isochron is a straight line describing how a particular isotopic ratio changes in response to radioactive decay. For instance, the abundance of 143Nd in rocks increases upon decay of their 147Sm content. When several minerals precipitate from the same magma, their 143Nd/144Nd and 147Sm/144Nd ratios form an isochron with a slope that can be converted into an age. An isochron typically requires that all of the synchronous systems involved formed with the same 143Nd/144Nd ratio and that no Sm or Nd were subsequently lost or added.