A hierarchy is easily represented in EN through tags. The issue is representing a poly-hierarchy i.e. one child tag with more than one parent tag. When using folder hierarchy, in Windows for example, the issue of poly-hierarchy representation is solved through shortcuts. You don't have to duplicate one particular file to have it in 2 different folders, you just have it in one folder and create a reference to it, i.e shortcut, within another folder(s). I am a teacher and I have this constant debate with myself about the best way to store my lesson plans. Every year, I teach the same content and my courses keep the same prefixes. However, my lesson plans do change, and the academic year changes, as well. To (well) organize my lesson plans, a poly-hierarchy is needed. If EN opts for "association", or what May called "networked", structure, then, imho, I think that the offered ability to nest tags is confusing. Since we organize ideas (and other stuff) through both associations and hierarchies, I think EN should offer this flexibility in both tags and Notebooks. One may say: "well, finding notes through search when they are tagged is easier". I say: "since EN search is powerful, you could still find your notes within Notebooks."... "you can still tag you notes that are inside a Notebook hierarchy. The issue of tagging a note with all parent and child tags can be solved with this simple app called Taggy. I know it's available for OSX but not sure for other platforms. Otherwise, I am grateful for such a neat program.