I'm realizing that I can't tell which of the classes in this example are custom and which are from the bootstrap file. I definitely understand that the "container" and "row" classes are referring to classes defined in the bootstrap file. And I'm pretty sure that the "nav" and "jumbotron" are bootstrap defined classes as well. The way that I look at the nav and jumbotron are like kind of being "content-specific" classes (reminds me of what some people refer to as "semantic" code). But ... is the "neighborhood-guides" class a custom class or a class defined in the bootstrap file? I don't see it defined in the main.css css file - but it seems odd that bootstrap would have a class by that name. Also - when I look at this page on bootstrap [http://getbootstrap.com/css/#grid] and the CSS page I don't see a reference to a 'neighborhood guides' class. Where is this class defined?