Thus the absolute configuration of (−)-glyceric acid must be the same as that of (+)-glyceraldehyde. Nitric acid[citation needed] oxidation of (+)-isoserine (3) gives (–)-glyceric acid, establishing that (+)-isoserine also has the same absolute configuration. (+)-Isoserine can be converted by a two-stage process of bromination[citation needed] and zinc reduction to give (–)-lactic acid, therefore (–)-lactic acid also has the same absolute configuration. If a reaction gave the enantiomer of a known configuration, as indicated by the opposite sign of optical rotation, it would indicate that the absolute configuration is inverted.