Similarly in Britain, typical notebooks (especially at school) use the B sizes. I think it's probably common across Europe. It means you can paste an A5 sheet

Similarly in Britain, typical notebooks (especially at school) use the B sizes. I think it's probably common across Europe. It means you can paste an A5 sheet handed out by the teacher into your B5 notebook, leaving a reasonable margin around it. Or you can fold an A4 sheet in half, and paste that in.