We study general two-body decays of primitive and non-primitive ¼-BPS dyons in four-dimensional type IIB string compactifications. We find a ``master equation'' for marginal stability that generalises the curve found by Sen for ½-BPS decay, and analyse this equation in a variety of cases including decays to ¼-BPS products. For ½-BPS decays, an interesting and useful relation is exhibited between walls of marginal stability and the mathematics of Farey sequences and Ford circles. We exhibit an example in which two curves of marginal stability intersect in the interior of moduli space.