Spoiler: a sharpie mark is about one ten-thousandth of an inch thick (depending on color, because size of the pigments!) An interesting use of this bit of triv

Spoiler: a sharpie mark is about one ten-thousandth of an inch thick (depending on color, because size of the pigments!) An interesting use of this bit of trivia: when machinists are trying to grind a block square, they may measure a slight deviation from 90 degrees - i.e. they see that each of the two opposite faces are parallel, but the angle they make is out of square. They have a parallelogram. If you draw it, you can see that if they put it back on the grinder, and shim one edge of the face contacting the table, when they grind the opposite-to-the-table face they can bring it closer to square because they'll be grinding a wedge off the top face. They use a quick swipe of a sharpie marker as a shim.