The Parallel Pen is so named because the ink flows through two parallel plates.  As you can see from the photos, you can use it as a standard broad nib or as a fine line writer.  It comes in four sizes, each with its own specific colour cap.

It is a great tool.  The preferred sizes are the two largest:  3.8mm Green and 6.0mm Blue.

You can purchase ink cartridges for them in a variety of colours.  Do so... but SAVE the empty cartridges.  You can fill them with your own ink.  Lisa Engelbrecht actually fills the barrel directly with FW Acrylic ink.  Amazing that she has had no issues.  Mind you, she uses them EVERY day and the ink doesn't have a change to dry and clog up the works. 

 

With thanks to Alice Young, here are instructions on how to refill the ink cartridges
and guidelines.