The single use rule
The single use rule is the idea that you should only use one notebook per function.
In other words, no two notebooks should be used for the same exact purpose at the same exact time.
Take my 2 notebooks for example. My primary notebook, is a journal used to capture my thoughts, record significant moments, random notes and tasks. Since I use this notebook as a regular reflection, tool it's important that my notes are legible and in a structured format.
On the other hand my second notebook is actually a sketchbook that I use to doodle and brainstorm. I could go back to reflect on my brainstorm doodles, but the sketchbook isn't a reflection tool for me. I use it to be more free with my handwriting and enjoy using different writing utensils.
So pretty much, I've got a structured notebook for my tradition journaling and a sketch notebook for my non-traditional doodles.
The single use rule. Simple. And structured.