Key idea: a black pearl needs two cells of straight room in one horizontal and one vertical direction. Anything that removes that room — the border, a ✕, another pearl, or a line crossing its path — forces the arm the other way.
New to the rule itself? Read black pearl rules first. In the diagrams, green lines are new deductions and red ✕ marks are ruled-out edges.
1 The corner opener
A black pearl in a corner has exactly one legal shape. Draw both arms immediately — four edges for free.
2 The edge pearl points inward
On the border, the arm perpendicular to the edge can only point into the grid. Only the direction along the edge is left open.
3 The second-row rule
A black pearl one cell from the border can’t point toward it: the arm would run out of grid after one cell. It points away — two cells inward. In the second row and second column, both arms are forced.
4 Arm locks
The first cell of every arm is passed straight through, so its two side edges are ✕. The edge opposite each arm is ✕ too, because the pearl turns. One drawn arm often produces three ✕ marks.
5 Adjacent black pearls point apart
Two black pearls side by side can’t be joined — the joining arm would pass straight through a black pearl. So in that axis each points away from the other, and the edge between them is ✕.
On the border this is even stronger: two black pearls stacked vertically against the top edge can’t both be legal, so you will never see that arrangement in a valid puzzle.
7 The blocked arm
If the neighbouring cell on one side of a black pearl already has a line running across it, the arm can’t pass straight through. The pearl must use the opposite direction.
White pearls can block arms too. Below, the three white pearls in a row must all run vertically, so the black pearl’s arm can’t enter the first one sideways.
8 Two diagonal white pearls
White pearls on both upper diagonals of a black pearl stop it pointing up. If it did, the cell above would be vertical, both white pearls would be forced vertical, and one of them would run straight down into the black pearl’s horizontal arm. That’s a contradiction, so the arm goes down.
This is a short what-if chain in disguise — once you’ve seen it a few times, you’ll recognise it at a glance.
9 The black-pearl sweep
On every new puzzle, before anything else, visit each black pearl and ask: which of my four directions still has two cells of straight room? If only one horizontal or one vertical direction remains, draw it. Then add the arm locks. On many Easy and Medium puzzles this sweep alone solves a third of the board.
Practise these patterns
Corner and edge black pearls appear constantly on 6×6 and 8×8 boards. For adjacent and diagonal patterns, try 10×10 Medium. Then move on to white pearl techniques.