AbstractConceptual, logical and dynamical restrictions on the criteria characterizing the mutual bonding of particles are examined. In accord with these restrictions, observables consisting of fixed linear combinations of the particles' position and conjugate momentum are used to construct appropriate “boundedness” criteria. As a result, comparable localization of relative position and relative momentum are obtained for mutually bound particles; neither excessively compact nor excessively diffuse localization can result for bonding to oc