2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/164785We characterize all coexistent pairs of qubit effects. This gives an exhaustive description of all pairs of events allowed, in principle, to occur in a single qubit measurement. The characterization consists of three disjoint conditions which are easy to check for a given pair of effects. Known special cases are shown to follow from our general characterization theorem.A paper with identical title is being published on the arXiv simultaneously by P. Busch and H.-J. Schmidt. These authors solve the same problem independently with a different method. In v2 the proof of the main result has been clarified, and some minor errors have been corrected. In v3 some typos have been corrected and a reference has been addedQuantum Physicstext