gives rise to an action on the matrix "X" in (). This does not give a well-defined action on primitive triples, since it may take a primitive triple to an imprimitive one. It is convenient at this point (per ) to call a triple ("a","b","c") standard if and either ("a","b","c") are relatively prime or ("a"/2,"b"/2,"c"/2) are relatively prime with "a"/2 odd. If the spinor ["m" "n"]T has relatively prime entries, then the associated triple ("a","b","c") determined by () is a standard triple. It follows that the action of the modular group is transitive on the set of standard triples.