Since radioactive decay is exponential with a constant probability, each process could as easily be described with a different constant time period that (for example) gave its "(1/3)-life" (how long until only 1/3 is left) or "(1/10)-life" (a time period until only 10% is left), and so on. Thus, the choice of and "" for marker-times, are only for convenience, and from convention. They reflect a fundamental principle only in so much as they show that the "same proportion" of a given radioactive substance will decay, during any time-period that one chooses.