DNA has four "letters" that must specify the 20 different amino acids that make up proteins. Combinatorially, using three DNA letters for one amino acid makes the most sense. (DNAi Location: Code > Reading the Code > Pieces of the puzzle > The genetic code)

Once the DNA double helix had been discovered, the next big challenge was to work out how the four letters of DNA could code for each of the twenty amino acids that make protein. The first question was how many DNA letters coded for each amino acid? If it was one DNA letter for one amino acid then you could only code for a maximum of four amino acids. Two letters in every possible combination could code for up to sixteen amino acids. Still, not enough. But three DNA letters provide more than enough combinations to code for all twenty amino acids. So three was the answer. It was a triplet code.

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