Each word of codon consists of 3 letters i.e., the codon is triplet. Thus we can say that each amino acid is specified by 3 nitrogenous bases in DNA/RNA. Since there are 4 nitrogenous bases which code for 20 amino acids, a single nitrogenous base is insufficient because through this only 4 combinations are possible. Similarly a combination of 2 nitrogenous bases (16 codons) are also insufficient for 20 amino acids.