Non-canonical base pairing

Non-canonical base pairs are planar, hydrogen-bonded pairs of nucleobases with hydrogen-bonding patterns that differ from those of standard Watson–Crick base pairs found in the classic double-helical structure of DNA. Although non-canonical pairs can occur in both DNA and RNA, they primarily form stable structures in RNA, where they contribute to its structural diversity and functional complexity. In DNA, such base pairs are typically transient and arise during processes like DNA replication.

Source: Wikipedia — Non-canonical base pairing (CC BY-SA 4.0)