The rule: the loop must turn 90° on a black pearl, and both segments leaving it must travel straight through the next cell. Each black pearl is therefore the corner of an “L” whose two arms are at least two cells long.
Anatomy of a black pearl
Think of every black pearl as having two arms: one horizontal and one vertical. The pearl chooses one horizontal direction (left or right) and one vertical direction (up or down) — so an unconstrained black pearl has exactly four possible shapes.
Each arm covers two edges: from the pearl into the neighbouring cell, and from that cell straight into the next one. After that the loop is free — it may keep going straight or turn.
What breaks the rule
Those are the only two ways to break a black pearl: not turning, or turning again too soon. The live error markers on our boards flag both instantly.
How position limits a black pearl
Because each arm needs two cells of room, a black pearl’s position on the grid often removes options before you’ve drawn anything:
| Where the pearl sits | Possible shapes | Why |
|---|---|---|
| Corner cell | 1 | Both arms must point into the grid. |
| Edge cell (not a corner) | 2 | The arm perpendicular to the edge must point inward. |
| Second row or column from an edge | at most 2 | An arm toward the near edge would run out of room after one cell. |
| Interior, two or more cells from every edge | up to 4 | All four directions fit — other clues decide. |
Black pearls together
Side by side: always pointing apart
Two orthogonally adjacent black pearls can never be joined: an arm from one would have to pass straight through the other, and black pearls must turn. So each sends its arm in that axis away from its neighbour. See adjacent black pearls for the full technique.
One cell apart: they may share an arm
If two black pearls sit in the same line with exactly one cell between them, they can share an arm: the loop runs from one pearl, straight through the middle cell, and turns on the other. This is legal because each arm only needs to pass straight through its neighbouring cell.
Black pearls next to white pearls
A white pearl directly beside a black pearl is automatically satisfied whenever the black pearl’s arm passes through it: the arm makes the white pearl go straight, and the black pearl itself provides the required turn next door.
Combinations of black and white pearls produce some of Masyu’s most satisfying deductions — such as a black pearl flanked by two diagonal white pearls. Those are covered in black pearl techniques.
Black pearl questions
Can the loop go straight through a black pearl?
No. The loop must make a 90° turn on every black pearl. Passing straight through one breaks the rule.
How long is a black pearl’s arm?
At least two cells. Each arm leaves the pearl and passes straight through the neighbouring cell; the loop may turn again in the cell after that, or continue straight.
Can two black pearls share an arm?
Yes. If two black pearls sit in the same row or column with exactly one empty cell between them, the arm of each can run through the middle cell to the other pearl, where it turns.
Can a black pearl sit next to another black pearl?
Yes, but they can never be joined directly. Each must send its arm away from the other.
Next, learn the complementary rule: white pearls.