a) The lactic acid forms two kinds of antimer with optical rotation, namely L (+)-lactic acid and D (-)-lactic acid, for the reason of different spatial arrangements of its asymmetric carbon atom. ”D” and ”L” refer to the configurations, where L is laevoisomer and D is dextroisomer. (+) Or (-) refers to the direction of optical rotation, where (+) is dextrorotation and (-) is laevorotation. There is not a definite relation between configuration and direction of optical rotation. D compound might not be dextrototation, and L compound might not be laevorotation.

b) The known forms of lactic acid in existence include L (+)-lactic acid (L-lactic acid in short), D (-)-lactic acid (D-lactic acid in short) and DL-lactic acid (racemic or mixed).