The Valley of Beta Stability is a set of atomic number/weight pairs
(isotopes) for which the balance of protons and neutrons does not
induce typical Beta Decay, because they have sufficient Binding Energy.
Natural isotopes tend to fall in this set, because if some other
form of radioactivity or nuclear reaction creates an isotope outside
this set, it will beta decay until it reaches this set.
As such, its existence is part of the theory explaining the
abundance of elements.