This was a quick experiment with a calligraphy pen called an Elegant Writer.
It was mentioned in Watercolor Without Boundaries, by Karlyn Holmans:
A wet brush pulled across the lines makes this lovely shading effect, and
supposedly once this layer is dry, it can be painted over without additional
streaking. I'll have to do some more experimenting to see how that turns out.
Anyway, the pens come in a pack of 4 for about seven bucks at Joann.