Isomers are compounds that have the same molecular formulas but different three-dimensional structures. The difference between isomers can literally mean the difference between life and death. L-dopa, shown on the left, reduces the symptoms of Parkinson's disease. Its isomer R-dopa, on the right, has no effect on the disease. There are three types of isomers. Structural isomers differ in the covalent partnerships between their atoms. Cis-trans isomers vary in arrangement of atoms around a double bond. And enantiomers are molecules that are mirror images of each other, like left and right hands.