2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/146057Positivity restrains the allowed domains for pairs or triples of spin observables in polarised reactions. Various domain shapes in ${1\over2}+{1\over2}\to{1\over2}+{1\over2}$ reactions are displayed. Some methods to determine these domains are mentioned and a new one based on the anticommutation between two observables is presented.Talk given by Xavier Artru at "DSPIN-07", XII Workshop on High-Energy Spin Physics, Dubna, Sept. 3-7, 2007, to appear in the ProceedingsNuclear TheoryHigh Energy Physics - Phenomenologytext