My lecture notes describe epimers as compounds which differ by
configuration at only one carbon and gives the example of D-erythrose and D-threose as being epimers.
However isn't that the same thing as diastereomers? My understanding is that epimers are supposed to be some kind of subset of diastereomers, however I am not sure how or why. I tried searching the web but I got more confused with one site saying that carbohydrates in open form can have two different chirality centres and still be epimers.
