 So the second fold I showed you was one where the two layers of beta sheets had their individual strands aligned. Again you will occasionally hear me making the mistakes because this is a difficult distinction to make. The complete layer is a beta sheet. Each individual component of the amino acid backbone is a beta strand. An individual beta strand is not stable it's just when you have multiple strands adjacent to each other that they form a stable sheet. That second fold is very common in immunoglobulins in your immune system and here is an example of an x-ray structure where you have four of them. Those beta sandwich folds as we call them tend to occur in the outer parts of antibodies up here. So they're very common regions for binding things where you have an antigen on a cell. An antigen is the part that the antibody binds while the antibody itself is the part of our immune system that's recognizing them. There are a couple of reasons for that but it turns out that beta sheets are frequently very efficient binding domains. If you're on the one hand we have all these potentially unpaired hydrogen bonds along the last strand in each sheet right? But then you also have after all the strands in one sheet all the loops connecting them this typically creates a very nice small cavity or something where you can either bind a compound or having this cavity itself with the loops evolved to bind a particular other protein. So it's no coincidence that these are very common in your immune system and again this particular fold we tend to call beta sandwich. Now in practice things are not crystal clear you might have seen that in the previous slide it's definitely not the case that one of them has the angle zero degrees between the strand and the other one is 90. So how do we classify these? Well half of it is based on humans and humans make errors. In many cases we tend to classify things more based on evolution because you could argue that if there are many immunoglobulins it makes sense that we group all of them in one fold even if some of them have evolved to start to deviate a bit. While in other cases if we had many of the fatty acid binding proteins in the extreme case one of those folds might have evolved to be have almost the same angle as the immunoglobulins but it still makes sense to group them evolutionary where they came from and that's why occasionally we'll see these things quite close to each other.