Nucleotide sequences carry the information that determines amino acid sequences, or, expressed the other way round, the amino acid sequence of a protein is encoded by a nucleotide sequence. The link between the two types of molecules that guarantees the transmission of the information is called the genetic code. One nucleotide is obviously not enough, since only four amino acids could be encoded this way. That is not only enough but already too much. Genetic experiments and physicochemical measurements suggested a triplet code.
All codons are of equal length.
The answer can be found by a simple consideration. In an overlapping code would an amino acid in a protein have an influence on the choice of the following amino acid. An amino acid that would be encoded by AAA, for example, could in a strongly overlapping code only be followed by an amino acid encoded by AAX, i.e. the first amino acid had always to be followed by one of just four different amino acids. The full potential of the 20 amino acids could thus only be used in a non-overlapping code. Already in 1957 were enough experimentally determined amino acid sequences available to evaluate neighbourhood frequencies.