Chemical groups added as modifications to purine and pyrimidine rings to create the 18 “universal” tRNA modifications are shown, with arrows indicating the atom of the ring that is modified and common abbreviated name for each modified nucleotide indicated in parentheses. The C1-N6 and C4-N3 bonds of the purine and pyrimidine rings, respectively, are highlighted in red to indicate that the bonding order depends on the identity of the nucleotide base, with single bonds in the case of G and U, and double bonds in the case of A and C.