This database covers 382 (4 complementary and 378 non-complementary) base pairs that can be formed, regardless of whether their existence has been confirmed. These base pairs are classified into 12 types according to the Leontis-Westhof classification (Leontis & Westhof. RNA, 7, 499-512 (2001)). They are also classified according to the number of real H-bonds and CH-bonds involved and the co-planarity of the bases. Base pairs formed only by CH-bonds, bifurcated base pairs, water-mediated and metal-mediated base pairs are not involved in this database.