This figure shows the core ring structures of penicillins (top) and cephalosporins (bottom). They both contain a ring that is joined to the common beta-lactam ring (shaded in pink), but the structure of this ring differs between the two classes: the penicillins have a sulphur (S)-containing 5-membered ring with no double bonds; the cephalosporins have a sulphur (S)-containing 6-membered ring with one double bond.