Megan Rhiannon has a really cool system for her commonplace notebook, which she has tested across different planners. I particularly like the way she adapted it for her Hobonichi Cousin, which she also uses for planning.
A commonplace book is a book where you can store everything—knowledge, quotes, observations, etc. I’ve seen people use it if they’re studying particular subjects, doing a lot of reading, collecting quotes, etc. The idea is that the act of writing these things down helps you better remember them and reference them. There are many different ways people do this—Megan color codes her entries with dot stickers.
This video explains how she does it and also includes a flip through. If you want to skip to the commonplace portion, it starts around the 17:00 mark.
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