title: Real world examples of tree structures
body: <p>I'm looking for some examples of tree structures that are used in commercial/free software projects, modern or old. I can see examples on wikipedia, but I am looking for more concrete examples and how they're used. For example primary keys in databases are (from what I've read) stored in BST structure or a variation of the BST (feel free to correct me on this)</p>
  
  <p>My question isn't limited Binary Search Trees (BSTs), it can include any variation such as red-black, AVL and so on.</p>