I know the A-series is more common, but I sometimes prefer the B-series. It's nice if, for example, you're looking for something between A4 and A5 (i.e. B5).
Individual images made by Wikipedia user Bromskloss (CC BY-SA 3.0)
B6 has become my go-to size notebook for journaling. Used to be a committed A5 guy but the slightly smaller B6 is just handier for most situations, while still being more generous than a “pocket” size.
For the life of me I don’t understand the weirdly proportioned “B6 Slim” and its popularity.
I think b6 slim love comes purely from the traveler's notebook. And even then that's a format most were putting up with rather than enjoying. But you know how things go when things get popular. People just start doing the popular thing, including for capturing that same aesthetic, and the b6 slim travelers notebook has a very particular aesthetic thanks to how long and tall it is
It's the same, the difference is the starting "0" size. For the A-series, the area is 1 m^2. For the B-series, the shorter side length is 1 m. The C-series is the geometric mean of the areas of the A- and B-series.
When I was a student, all my notebooks were either A4 or B5, these days even A5 is too big for me and I end up with A6 for most of my handwriting stuff.
Never seen a C-series paper though, didn't even know it existed.