The STAEDTLER triplus fineliner is a water-based pen for fine lines, with a line width of 0.3mm. A metal guide protects the tip, and the barrel is triangular. Except for the six neon colors, the pen can be left with the cap off for two days and still write without drying out.
Besides single pens, there are sets of 4, 10, and 20 colors.
The thin line works well for outlining detailed patterns and filling in small areas.