Although amino acids are often shown in textbooks as the right-hand structure, they actually mostly exist as the left-hand structure. This table shows the abbreviations and single letter codes used ...

However, with only 20 amino acids, a triplet code would also suggest redundancy–in other words, more than one codon might correspond to the same amino acid, or there might even be "spare" or ...

The last universal common ancestor is the single ... letter designation is W). “[T]here is scientific consensus that W was the last of the 20 canonical amino acids to be added to the genetic ...

The genetic code is a set of three-letter combinations of nucleotides ... Although each codon is specific for only one amino acid (or one stop signal), the genetic code is described as degenerate ...