The reason the degree, and not the gon, has persisted as the commonly used unit of angle is that 1/3 of a quadrant is the arc sine of 1/2 and is thus a more commonly used angle than 1/5 or 2/5 of a quadrant, whose sine, cosine, and tangent are irrational. A hex nut's sides are parallel and it can be turned with an ordinary wrench, whereas a pentagonal nut cannot and it is relegated to fire hydrants. 1/3 of a quadrant is 30 degrees, but not an integral number of gons.
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