The ability of Watson and Crick to solve the structure of the B DNA duplex was in large part due to model building and understanding base pairing in addition to knowing that the structure was a right handed helix with specific helical repeat. Key to being able to propose a structure was making use of Chargaff's rule that there was a 1:1 molar ratio of A to T and G to C, indicating that they existed as AT and GC pairs. Then it was a matter of understanding how A could pair with T and G with C. Molecules can associate by a number of different mechanisms, such as electrostatic (opposite charges attract), dipole dipole interactions, van der Waals interactions (dipole induced dipole), and H-bonding. Among these types of interactions, H-bonding is the most specific and most likely to explain the specific AT and GC base pairing. H-bonding involves the interaction of a polarized X-H bond (the H bond donor) where X is an electronegative atom such as oxygen O or nitrogen N with a negatively polarized lone pair on Y: (H-bond acceptor) where Y is also O or N to form a colinear X-H---:Y H-bond (see Figure 1). To understand how two bases could interact with each other, however, they first had to know their structures and this is where they faced a major problem. Nowadays the bases are drawn in their Watson Crick forms, but back then it was uncertain where the hydrogen bonding hydrogens were. This is because molecules containing certain functional groups of the type X=Y-ZH can exist in equilibrium with H-X-Y=Z. These two forms are known as proton tautomers. Even a simple functional group such as an amide can exist with many multiple tautomeric forms which could interact with other molecules in many different ways as illlustrated below for the molecule formamide (Figure 2). As one can see each tautomer has a different set of NH or OH bonds pointing in different directions.