  Two quantum events, represented by positive operators (effects), are
coexistent if they can occur as possible outcomes in a single measurement
scheme. Equivalently, the corresponding effects are coexistent if and only if
they are contained in the ranges of a single (joint) observable. Here we give
several equivalent characterizations of coexistent pairs of qubit effects. We
also establish the equivalence between our results and those obtained
independently by other authors. Our approach makes explicit use of the
Minkowski space geometry inherent in the four-dimensional real vector space of
selfadjoint operators in a two-dimensional complex Hilbert space.
