D and L (Relative) Configuration

Before the CIP $R/S$ system was developed, chemists described the configuration of a stereocentre relatively, by comparing it to an arbitrarily chosen reference compound rather than by any absolute priority rule. This is the $D/L$ (or Fischer) system.

The reference compound is glyceraldehyde. In a standard Fischer projection of glyceraldehyde (drawn with the most oxidised carbon at the top and the carbon chain running vertically), the isomer with the $\text{--OH}$ group on the right-hand side of the stereocentre is designated D-glyceraldehyde, and the isomer with $\text{--OH}$ on the left is designated L-glyceraldehyde. Any other chiral compound is then assigned $D$ or $L$ by relating its Fischer projection, group-by-group, to this reference -- a compound whose configuration at the analogous carbon corresponds spatially to D-glyceraldehyde is called $D$; the mirror-related one is $L$.

$D/L$ and $R/S$ are independent systems that need not correlate. $R/S$ is assigned purely from CIP priorities and has nothing to do with any reference compound, so a $D$-labelled compound can be either $R$ or $S$ depending on what groups happen to be attached (for D-glyceraldehyde itself, it happens that $D$ corresponds to $R$, but this correspondence does not carry over automatically to other compounds with different substituent priorities). …