 So I mentioned that it's advantageous to put something hydrophobic every seven residues. The reason for that is that you end up with this. So the red ones here are leucines. It's not every turn, but what now happens is that every second turn we're going to have a residue that's hydrophobic, and if I do that in the second helix too. Typically, it's the same sequence here. So I have the same helix expressed twice. These two helixes will now love to pair up with each other. Those hydrophobic compounds should be turned away from water, so they will face each other. This is also a very common motif in the sequence, and we'd call it leucine zippers, zipper like the zipper here. The reason for that is that those hydrophobic leucine is slightly larger than alanine, right? And it's significantly more hydrophobic. It won't to collapse away from water. So this will literally make the two helixes zip up like a zipper. And that's why it's advantageous to have them at 3.5 instead of 3.6, because if it was actually 3.6 we would gradually turn and occasionally have the leucine's face water, which would be bad. We can do some things better. What if you take some residues here? Maybe not every turn, but every 5-10 turns and put a cysteine there. Remember what cysteines can do? If we oxidize them, they can form a disulfide bridge. So if you now compare this nice forming structure with an ability to form disulfide bridges once you've put the helices next to each other, we can end up with a super strong structure, and then it becomes almost impossible to break this up. Still, two helices. We're talking about the radius here. That's well below 1 nanometer. What if I try to make something larger? This is now one pair of helices. If you ever look at a piece of jarn or maybe a piece of rope, you will see that it's in hierarchical structure. So first you might have two fibers twisted, and then there are two fibers like that, and then there are two fibers like that. So that two multiplied by two, multiplied by two, essentially. What if I take one of these pairs here, and imagine that I had another similar pair here? Now I have four helices. These two pairs I could start to twist and form a coiled coil, essentially. We usually don't use that term.