Masyu

Masyu is a loop-drawing logic puzzle played on a grid containing white and black circles (pearls). The player must draw a single continuous loop that passes through every pearl, following specific turning rules: the loop must turn 90 degrees on every black pearl and travel straight through it on both sides, while on every white pearl the loop must go straight through but must turn on at least one of the two neighboring cells. The loop cannot cross itself or branch. The elegant interplay between the two pearl types creates puzzles of surprising depth.

Rules

1. Draw a single closed loop (no endpoints) on the grid using horizontal and vertical segments along the grid lines, passing through cell centers.
2. The loop must pass through every circle (pearl) on the grid.
3. Black pearls: The loop must make a 90-degree turn on the black pearl, and must travel straight for at least one cell on both sides of the turn.
4. White pearls: The loop must go straight through the white pearl (no turn), but must turn on at least one of the two cells immediately before or after it.
5. The loop cannot cross itself or pass through any cell more than once.
6. Not every cell needs to be used — the loop only needs to satisfy the pearl constraints.

Strategies

• Black Pearl Edges: A black pearl on the edge of the grid forces the loop's direction, since it must turn on the pearl and go straight for at least one cell in each direction from the turn. A black pearl in a corner has only one possible turn configuration.
• White Pearl Edges: A white pearl on the edge forces the loop to run parallel to that edge (since it must go straight through), and forces a turn on the adjacent interior cell.
• Adjacent Black Pearls: Two black pearls next to each other create strong constraints — the straight segments extending from each turn must not conflict.
• White Pearl Chains: Multiple white pearls in a line must all be traversed straight, but the required turns on neighboring cells can quickly restrict possibilities.
• Near-Complete Regions: When most paths in an area are determined, the remaining connections are often forced by the loop closure requirement (no dead ends, no crossings).
• Forbidden Patterns: Learn to recognize configurations that would force the loop to close prematurely or create a dead end — these help eliminate incorrect paths early.

Masyu was created by the Japanese puzzle publisher Nikoli. The puzzle first appeared in Nikoli's magazine "Puzzle Communication" in 1999 under the original name "Shinju no Kubikazari" (真珠の首飾り), meaning "Pearl Necklace." The name was later shortened to "Masyu" (ましゅ), which is a playful abbreviation derived from the original.

The puzzle was designed by Nikoli editor Yoji Ojima, who was experimenting with loop-drawing puzzles — a category that Nikoli had been developing since the creation of Slitherlink in 1989. Masyu refined the concept by introducing two types of constraints (black and white pearls) that interact in elegant ways. The dual-constraint system was considered a significant innovation in puzzle design.

Masyu quickly became one of Nikoli's most popular puzzles and was adopted internationally through the World Puzzle Championship, where it has been a recurring puzzle type. The puzzle was popularized in the English-speaking world through puzzle magazines, online communities, and Simon Tatham's Portable Puzzle Collection (under the name "Pearl").

The puzzle's enduring appeal comes from its visual elegance and the satisfying way local constraints (individual pearl rules) combine to determine the global solution (the entire loop). Constructing high-quality Masyu puzzles is considered an art form — the best puzzles use a minimum of pearls to define a unique loop, creating an "aha moment" when the solver discovers how the sparse clues interconnect. Today, Masyu appears in puzzle competitions, books, and digital platforms worldwide, and is widely regarded as one of the finest loop-based logic puzzles ever created.