This is the carbon that we have to worry about. And if this hydroxyl group on the final stereogenic carbon points to the right side, then that is the deisomer, the Dnamer. But if it points to the left side, so that is called the L monosaccharide, in this case, the L erythros. So the symbol D or L designates the absolute configuration of the stereogenic carbon, the chiral carbon, that is farthest away from the aldehyde or ketone group. And in this particular case, it happens to be the aldehyde group. Now, in deisomers, the oh points to the right side, and in L isomers, it points to the left side.