Step 4 of 5
Link
The signs between cells: = means same, × means opposite.
The rule of thumb: An = or × sign with one side filled tells you the other side.
A sign on the border between two cells ties them together. = says they hold the same symbol; × says one is a sun and the other a moon.
A sign on its own tells you nothing. The moment one of its cells is filled, the other follows. That new symbol may start a pair or complete a count, so Link often gets the line rules moving again.
From here on the boards have signs, and these practices can’t be solved without reading them.
Where to look: Signs with exactly one side filled. Whenever you place a symbol, check whether it touches a sign.
Worked example
A real position from a board that needs Link. Purple: what the reasoning rests on. Dashed: the cell in question. Green: what it proves (“this cell” in the text).
Every Easy and Medium board here is now within reach. One technique left: the one Hard boards need. Step 5: Lookahead →