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It occurs to me (and this goes for other kinds of organizing too) that notebooks are for people and tags are for computers. In other words: notebooks allow a person, at a glance, to get to the right area, which is useful if you have only a modest number of notebooks and, perhaps, a modest number of notes. Tagging is somewhat more powerful, because it harnesses the power of search. In other words, notebooks enable search by your eyeballs, whereas tags enable search by your computer, an indispensable feature as you accumulate lots of notes.