Pen manufacturer Aechy makes these unusual dual-tipped pens. One end is a conventional fine-tip rollerball, but the other end is essentially a rolling stamp that lays down a pattern. (I guess this is for folks who don't have the time to draw dotted lines, or for whom inconsistently wavy lines are intolerable.)
Strangely, you cannot specify which color you want with which pattern; while the wavy line and dotted line patterns each offer two colors, if you want the centerline pattern, I hope you like yellow.
They're interesting, but nowhere near as cool as that dotted line tool from the 1930s with swappable gears for different patterns:
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