Masyu

Masyu, literally "pearls" in Japanese, is a logic puzzle published by the puzzle company Nikoli. The grid contains white and black circles placed in some cells. The objective is to draw a single, continuous loop along the grid lines that passes through all the circles and adheres to color-specific constraints.

Rules: The loop must be closed, non-self-intersecting, and pass through every marked circle. White circles require the loop to pass straight through the circle; the path cannot turn on a white circle. Black circles require the loop to make a turn at the circle; the path cannot pass straight through a black circle.

Solving involves deducing the path by exploiting local constraints around circles and propagating implications across the grid. White circles fix straight-through segments, while black circles force corners and can influence neighboring cells. The challenge is to produce a consistent, single loop that satisfies all circle constraints without violations elsewhere on the grid.

Puzzles vary in size and complexity, from small grids with a handful of circles to larger grids containing many pearls. Masyu emphasizes logical deduction, pattern recognition, and constraint propagation, rather than luck or trial-and-error. It appears in puzzle books, magazines, apps, and online collections, where solvers share techniques and annotated solutions.