Collapsible ones are extremely practical as you also get many extra functions in the bargain with, for example, degrees. That they are also easy to carry in the pocket of your notebook, in a pencil case or standing in a pen cup without being in the way is another plus. Then we have partly in a folding ruler in plastic and also in a more durable material such as aluminium.
There is also a smart model where a plastic ruler is combined with index tabs, quick marking in a book.
In addition, there is a wide range of rulers that are really decorative. Partly in combination with a stencil but also with a clip. Nice way to mark where you are in the book. It is worth mentioning that there is a more pared-down variant that aims primarily at the clip function and with a ruler.
For finally affordable and cheap in plastic and a model with a stencil with the sight set on Bullet Journaling.
With us, as I said, you will mainly find smart models that are practical to have in notebooks or pencil cases. Always ready to have at hand!