Specificity of base pairing is determined by the number of hydrogen bonds available. Recall that he standard forms of C and G pair with three H-bonds and the standard forms of A and T pair with two.

    The rare form of C' has two H-bonds, and pairs with the alternative purine, A. Likewise, the rare form of T' has three H-bonds, and pairs with the alternative pyrimidine, G. By a slightly different mechanism, the rare forms of G and A force non-standard numbers of H-bonds on T and C, respectively.

    Thus, the rare tautomers pair with the alternative ("wrong") purine or pyrmidine: C' with A, T' with G, G' with T, and A' with C.