With his tetrahedral carbon models van’t Hoff explained the mysteries of known optical isomers possessing stereogenic centers and predicted the existence of chiral allenes, a class of molecules that would not be observed for another sixty-one years. Symmetry operations that involve inverting an odd number of coordinate axes interconvert mirror-images. Like printed words, only a small fraction of molecules are achiral. Verbal and pictorial notation for stereochemistry are discussed.