Well, from organic chemistry we know that isomers are basically those molecules that contain the same exact molecular formula. So they have the same exact number of atoms and the same type of atoms, but they differ in their arrangement of atoms in the three dimensional space. Now, we have two types of isomers when it comes to sugars. We have dirty isomers and we have enantomers. So just a moment ago, we said enantomers are those isomers that are mirror images of one another. Now, if it's not an enantomer, then it is a dioisomer.