An optical isomer can be named by the spatial configuration of its atoms. The  system (named after Latin dexter and laevus, right and left), not to be confused with the d- and l-system, see above, does this by relating the molecule to glyceraldehyde. Glyceraldehyde is chiral itself and its two isomers are labeled  and  (typically typeset in small caps in published work). Certain chemical manipulations can be performed on glyceraldehyde without affecting its configuration, and its historical use for this purpose (possibly combined with its convenience as one of the smallest commonly used chiral molecules) has resulted in its use for nomenclature. In this system, compounds are named by analogy to glyceraldehyde, which, in general, produces unambiguous designations, but is easiest to see in the small biomolecules similar to glyceraldehyde. One example is the chiral amino acid alanine, which has two optical isomers, and they are labeled according to which isomer of glyceraldehyde they come from. On the other hand, glycine, the amino acid derived from glyceraldehyde, has no optical activity, as it is not chiral (it's achiral).
The  labeling is unrelated to (+)/(−) it does not indicate which enantiomer is dextrorotatory and which is levorotatory. Rather, it indicates the compound's stereochemistry relative to that of the dextrorotatory or levorotatory enantiomer of glyceraldehyde. The dextrorotatory isomer of glyceraldehyde is, in fact, the  isomer. Nine of the nineteen -amino acids commonly found in proteins are dextrorotatory (at a wavelength of 589 nm), and -fructose is also referred to as levulose because it is levorotatory.  A rule of thumb for determining the  isomeric form of an amino acid is the "CORN" rule. The groups
:COOH, R, NH and H (where R is the side-chain)
are arranged around the chiral center carbon atom. With the hydrogen atom away from the viewer, if the arrangement of the CO→R→N groups around the carbon atom as center is counter-clockwise, then it is the  form. If the arrangement is clockwise, it is the  form. As usual, if the molecule itself is oriented differently, for example, with H towards the viewer, the pattern may be reversed. The  form is the usual one found in natural proteins. For most amino acids, the  form corresponds to an S absolute stereochemistry, but is R instead for certain side-chains.