We have this peptide here, amino acid number three. And finally we have this peptide here and amino acid number four. And so what we see is if this is our amino acid that contains the NH group, that it will interact with the co group, with the co group of the amino acid that is found four units ahead of that particular amino acid. And this is always true for the alpha helix. So it's the NH group of one amino acid that interacts with the co group of the amino acid that is found four units ahead of that amino acid. And the reason this takes place is because these are the groups that are found in closest proximity and are able to interact strongly.