A pen you can refill

A pen worth keeping takes a refill or a converter. The rest of the decision is balance in the hand and a line you can read tomorrow.

Fine nibs suit small notebooks. Medium nibs suit letters. A rollerball is the practical cousin when you do not want to think about ink.

Buy one, write with it for a week, and only then add another. A cup of twelve pens is a hobby. One reliable pen is a practice.