Adam Akhtar shares a clever tagging system for physical notebooks:
“[...] notebooks are hard to organize your ideas. You either split your notebook into several sections for each 'category' and end up wasting valuable pages in the quieter sections or you just write your ideas as they come along making them hard to find later on.
If this sounds familiar then you are going to love this little hack I was taught here in Japan by a friendly salariman. It's a little messy, and not something I'd use all the time but for the right subject could come in handy.”
May implement this idea in my own notebooks soon.