We can also have the alpha D ribofiorness, but only the beta Dribor nose is shown for simplification purposes. Now, unlike six member rings, which exist in the chair confirmation form, five member rings, five member sugar molecules exist in a puckered form we call the envelope form. So why we call it the envelope form? Well, simply because it looks like a simple envelope. So on an envelope, the four corners lie along the same plane. So these coroners are known as coplaner.