Tartaric acid can be taken as an example.
It has two asymmetric carbon atoms so using the 2n formula there should be a maximum of 22 = 4 stereoisomers. These can be represented as:
I and II i.e., threo forms are mirror images of each other and are enantiomers. But in erythro form, the plane of symmetry is present. Thus, it does not show optical isomerism. Therefore, tartaric acid exists only in three isomeric forms of which one is meso (optically inactive) and the other two are optically active, i. e., d (or) (+) and l or (-).
