You can buy one, or you can make one. Folded pens are a recent development in the world of calligraphy, where product development cycles take centuries. The nib is a folded piece of metal with a curved edge.
Here it is full-on and sideways:
The inside of the fold acts like an ink reservoir.
Varying the angle of the pen and the direction of the stroke gives wild, brush-like results. You can choose to be very controlled or very loose. The pen takes some getting used to – this is the first time I’ve ever tried one. Taking the time to experiment is worth it.
Allow the pen to run out of ink and then dip it quickly in another color for a rough-and-ready transition. Here, I began with Pilot Iroshizuku ku-jaku and then went to momiji.
Noodler’s Eel Black pretends to be sumi ink in this sketch sample. Here, I dipped the pen in water to dilute the ink that was already in it. That made a wash-like effect possible.
The paper has a little texture, so the raggedy edges of each stroke are more pronounced.