Positivity reduces substantially the allowed domain for spin observables. We briefly recall some methods used to determine these domains and give some typical examples for exclusive and inclusive spin-dependent reactions.
Positivity restrains the allowed domains for pairs or triples of spin observables in polarised reactions. Various domain shapes in ${1over2}+{1over2}to{1over2}+{1over2}$ reactions are displayed. Some methods to determine these domains are mentioned a
nd a new one based on the anticommutation between two observables is presented.
