An enantiomer can be named by the direction in which it rotates the plane of polarized light. Clockwise rotation of the light traveling toward the viewer is labeled (+) enantiomer. Its mirror-image is labeled (−). The (+) and (−) isomers have been also termed d- and l- (for dextrorotatory and levorotatory); but, naming with d- and l- is easy to confuse with - and - labeling and is therefore discouraged by IUPAC.