Octagon-Cut

Take a square, cut all four corners off evenly, and you have an octagon: eight sides, no acute angle anywhere on the outline. It reads as a square from a distance and reveals its geometry close up, which is a useful trick in a shape that has no point to break.

That durability is the practical argument. There is no thin corner to guard, so an octagon tolerates a simple mounting and everyday wear better than the square it came from. Standard millimeter sizes, seating cleanly in a plain four-prong head.