If this would have been the L, this would have pointed to the left side and we would have had this L erythro. So these are two examples of dirtyisomers. So, when two sugars have identical molecular formulas, have different arrangement of atoms and are not enantomers, they're not mere images, then they must be dieoisomers. Now, a specific subgroup, a specific subcategory of dialisomers, are epimers. So what exactly is an epimer? Well, two molecules are said to be epimers if or two sugars are set to be epimers if they differ in stereo chemistry only at a single chiral carbon atom.