Labels for vitamins or natural skincare often list ingredients with letters "D", "L", or "DL" preceding the chemical name of an ingredient. For example:

• D-alpha tocopherol is natural Vitamin E
• DL-alpha tocopherol is synthetic Vitamin E
• L-ascorbic acid is natural Vitamin C

As might be guessed from the short list above, a D or L prefix may be applied to a natural ingredient, but the DL prefix indicates that the ingredient is synthetic.

Many natural ingredients can rotate polarized light at a specific angle toward the right. In this case, the ingredient is labeled with a D, for dexter, Latin for "right". If polarized light is rotated at a specific angle toward the left, the ingredient is labeled with an L, laevus, Latin for "left". Less commonly, the left rotation is labeled S, for sinister, another Latin word for "left", or R, for rectus, another Latin word for "right". Whether or not your Latin is rusty, the key concept is easy to understand: the prefixes don't indicate whether the material is natural, but whether it is right-handed or left-handed; both cases occur in natural ingredients, with amino acids likely to be naturally L and sugars likely to be naturally D.

The phenomenon of right-handed and left-handed versions of the same molecule, identical except for the direction in which they rotate light, is caled chirality (from the Greek kheir, "hand").

When chiral ingredients are made synthetically rather than being derived from a natural source, half of what is produced will have the left-rotating form and half will have the right-rotating form; such a product is labeled "DL", indicating that both the right-rotating and the left-rotating version are present. The two enantiomers are mirror images of each other, just as the right hand is the mirror image of the left hand.

In nature, optically active molecules are D or L but not both; synthesis produces a racemic mixture of chemically identical molecules, differing only in their optical activity, some of which match the natural form and some of which are its mirror image. In many cases only the natural enantiomer (the naturally-occuring left-handed or right-handed form) is biologically viable, so whatever activity it may have in an organism does not take place with the opposite enantiomer; using the wrong-handed form can be unproductive. Sometimes the non-naturally-occurring enantiomer is toxic; using the wrong-handed form can be dangerous.

To see a better illustration and read a badly-spelled but really interesting suggestion that this right-handedness or left-handedness of molecules has to do with the directions of stars' rotation, start at this NASA web page.