here), some compounds have fewer. When the three groups on one chiral atom are
the same as those on the other, one of the isomers (called a meso form) has a plane
of symmetry, and hence is optically inactive, even though it has two chiral carbons.
Tartaric acid is a typical case. There are only three isomers of tartaric acid: a pair of
enantiomers and an inactive meso form. For compounds that have two chiral atoms,
meso forms are found only where the four groups on one of the chiral atoms are the
same as those on the other chiral atom.