The answer is; codon, codon, anticodon
There are about 64 different combinations of three nucleotide bases into codons. The combinations, however, only code for 20 amino acids hence some codons are redundant. When the mRNA forms a scaffolding with the ribosome, the tRNA with the right anticodon (sequence complementary to the codon on the mRNA) to the codon on the A site of the ribosome, leaves its amino acid at the site of chain elongation (P-site) and exits at the E-site.
