A device invented in the late nineteenth century by Sir Francis Galton to demonstrate error distributions and the normal distribution in particular.
The device consists of a vertical board to which pins are attached in a triangular pattern such that each set of five pins forms a quincunx – four pins at the corners of a square and the fifth one on the middle. The lowest row of pins ends in a series of rectangular slots. Balls are dropped through a funnel centered at the top of the quincunx, which then follow a random path between the pins to land into one of the slots at the bottom. As more and more balls are dropped, the height of ball columns in the slots approaches the characteristic bell curve of the normal distribution.
The Quincunx was used as a classroom tool to help students grasp the concepts of random process distribution.