The key difference between a chemical topologist's graph and a chemist's structural diagram is that the former does not consider molecular rigidity or geometry; when considering its topology, a molecular graph can be distorted arbitrarily provided the bonds are not broken or pass through one anotherall other transformations are valid, including the stretching of bonds and distortion of bonding geometry to an unphysical extent. Using this approach, we can distinguish between the topological properties of different classes of molecular isomers using the terms "homeomorphic" (same atomic connectivity) and "isotopic" (same molecular graph). Constitutional isomers (e.g., of pentane, Fig. ) are neither homeomorphic (different connectivity) or isotopic (graphs cannot be manipulated to become identical). Stereoisomers that are distinguished by differences in their arrangement of atoms in space due the geometric properties of atoms (Figure ) are homeomorphic (same connectivity) and isotopic (can be deformed into one another)they are topologically identical as the difference between the isomers is lost once their Euclidean properties are relaxed. In 1961 , Wasserman and Frisch recognized that some molecular structures cannot be reduced to a planar molecular graph, in that there is no two-dimensional projection of their three-dimensional structure in which no edges cross one another, and that such structures have isomers to which they are homeomorphic (same atomic connectivity) but not isotopic (different topology). This observation was inspired by Wasserman's synthesis of a molecular catenane in which two molecular rings are joined like links in a chain, which are topological isomers of the separated components (Fig. ). Other classes of molecules that display topological isomerism have since been reported, including Mobius structures , knots and covalent systems in which bond crossings are enforced . Wasserman and Frisch also highlighted that catenanes could exist as topological enantiomers if both rings are "oriented" due the sequence of distinguishable atoms embedded in their molecular graph (Fig. ) . Such catenanes have since been referred to simply as "topologically chiral", alongside Mobius ladders and some knots, and interlocked molecules containing multiple crossing points , which display topological enantiomerism independent of their constitution.