This was designed by Nikoli to be even more cultural independent than similar puzzles: It contains no numbers or letters, just colored disks as clues.
An unsolved puzzle consists of a rectangular grid. Some squares contain a white or black circle.
The goal is to draw a closed loop that satisfies the clues.
The loop cannot turn in white circles, and must turn 90° in black circles.
The loop must turn 90° in at least one of the cells before or after a white circle.
The loop must continue straight in both of the cells before or after a black circle.
The puzzle can also use a hexagonal grid.