  In the standard model, several forms of quark mass matrices which correspond
to the choice of weak bases lead to the same left-handed mixings $V_L=V_{CKM}$,
while the right-handed mixings $V_R$ are not observable quantities. Instead, in
a left-right extension of the standard model, such forms are ansatze and give
different right-handed mixings which are now observable quantities. We
partially select the reliable forms of quark mass matrices by means of
constraints on right-handed mixings in some left-right models, in particular on
$V^R_{cb}$. Hermitian matrices are easily excluded.
