Thymine is one of the pyrimidine bases. Pyrimidine is a 6-membered aromatic ring with two N-atoms in positions 1 and 3. Thymine is derived from pyrimidine by twofold hydroxy substitution in positions 2 and 4 and methyl substitution in position 5. Both of the N=C-OH groups tautomerize to NH-C=O. Note the conformation of the methyl group.