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. And it doesn't matter which carbon atom it is, as long as it's a chiral carbon atom, it's stereogenic. So, to see what we mean, let's compare D glucose and Dmannose. Again, these are both D. And what that means is these two green hydroxyl groups found on a final stereogenic carbon will both point to the right side of the board.