Base pairing is an important feature of nucleic acid structure with fundamental implication for the flow of the genetic code. Base pairing is realized via distinct hydrogen bond edges and results in canonical and non-canonical geometries as described in Leontis-Westhof nomenclature . We decided to analyze geometries of “mismatched” DNA base pairs, i.e. base pairs in non-Watson-Crick topologies by solving of crystal structures with mismatched pairs in 18-mer DNA duplexes. Previously crystallized 18-mer DNA oligonucleotide contained double thymine mismatch in the central positions , now we obtained nine additional variants covering both canonical and non-canonical base pairing. The refinement and validation stages of solving all our structures benefited from using the NtC dinucleotide classes (https://dnatco.datmos.org/) . Firstly, we used NtCs as refinement targets and secondly for annotation of solved structures. We present a comprehensive structural annotation of the base pairs in the ten solved 18-mer duplexes.