Pairing of complementary nucleobases is the most famous example of molecular recognition. However, it has long been known that except 9-methyladenine (9-mA) and 1-methylthymine (1-mT), all other binary combinations of nucleobases do not form base pairs under plausible prebiotic conditions, e.g., in water or in the solid state. It is surprising that nucleobases would have been incorporated into DNA if they were unable to self-assemble prior to their attachment. Here we show how the formation of an elusive base pair between 9-methylguanine (9-mG) and 1-methylcytosine (1-mC) is possible in the solid state via Watson-Crick hydrogen bonding. Molecular recognition of 9-mG:1-mC as well as 9-mA:1-mT base pairs was observed by X-ray diffraction monitoring during heating their 1:1 solid mixtures, while all other binary mixtures failed to give base pairs. To demonstrate the selectivity of complementary nucleobase pairing, we showed how 9-mA and 1-mT self-assembled in ternary mixture containing also 1-methyluracil (1-mU), and both 9-mA:1-mT and 9-mG:1-mC pairs self-assembled in quaternary mixture. The results presented here indicate the importance that the solid state as a reaction medium might have had as a prebiotic molecular selection tool towards compatibility as found in the DNA.