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.)
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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: