Masyu

Masyu (ましゅ, Mashu, IPA [maɕu͍]) is a type of path-drawing logic puzzle designed and published by Nikoli. It is played on a rectangular grid of squares, some of which contain circles; each circle is either "white" (empty) or "black" (filled). The goal is to draw a single continuous non-intersecting loop that properly passes through all circled cells.

Rules
Masyu is generally played on a grid of squares containing two types of clues: white circles and black circles. The goal is to draw a single continuous non-intersecting loop that satisfies the following properties:
 * It must pass through every circle.
 * The loop cannot turn on a white circle, but must turn on at least one of the adjacent cells in its path.
 * The loop must turn on a black circle, but cannot turn on either of the adjacent cells in its path.

History
The puzzle type was originally called Shiroshinju Kuroshinju (白真珠黒真珠 "White Pearls, Black Pearls"). The name Masyu reportedly derives from a misreading of the kanji for "pearl" (真珠 shinju) by Nikoli's president; the name caught on and was made the official name in Puzzle Communication Nikoli #103.

Puzzle Applications
Masyu is a standard grid logic puzzle genre; as such, its presence is often known from the beginning of the puzzle. Such puzzles often directly state that the puzzle is a Masyu and either expect solvers to look up the rules or provide the rules as plaintext within the puzzle.

In puzzles where the presence of Masyu isn't directly called out, it is very commonly clued in flavortext using the erroneous translation "evil influence" (derived from a literal translation of the unrelated Japanese term 魔手, also pronounced mashu).

Puzzlehunts often feature standard logic puzzle genres presented in unorthodox ways; as such, a puzzle may present a variant on the standard Masyu, changing the grid or tweaking the rules in a manner that creates a completely different puzzle with different deductions to be made.

Strategy
Deductions for a standard Masyu mostly revolve around its two types of circles and how they behave with respect to other circles, the edge of the grid, and extant sections of the loop. A gallery of such deductions follows.

Notable Examples

 * - Masyu, with the additional twist that it's played on a map of the London Underground.
 * - The meta for the Sleeping Beauty round, Masyu is clued using the "Evil Influence" cluing.