What pen do you use to take notes in the book?

I usually write down what the teacher says with a blue ballpoint pen. If there is any key point, I will write it down with a red ballpoint pen and write the key point in the upper right corner of the book. I will turn over the book when reviewing, and it will be clear what the key and difficult points are. Just use a ballpoint pen, because gel pens are too expensive. You don't need to be neat, just know yourself.