The invention relates to non-natural bases and base pairs that expand the normal DNA-based encoding system. Compositions herein may comprise at least one non-natural base that may interact with another base via a Watson Crick-type hydrogen bonding geometry and/or a Hoogsteen-type hydrogen bonding geometry. The bases may be used in a molecular entity, such as an oligomer or any other entity wherein the bases are attached to a backbone. For example, they may be comprised in DNA, RNA, or PNA, or a variety of other nucleic acid-type systems.