Step 5 of 8
Torches can't see each other
Every torch you place forbids a whole cross of cells. Use the whole cross.
The rule of thumb: Every cell a torch lights is a cell that can never hold a torch. Mark the whole sight line X.
Two torches may never shine on each other, so every cell a torch lights — its row and column until a pillar blocks the light — is a cell that can never hold a torch. Mark the entire sight line X. One placed torch can decide up to a dozen cells.
It also aims the other way: when you’re weighing a candidate cell, trace its sight lines. If a torch there would shine on one already placed, the cell is X. These practice boards stall without the markings — leave the sight lines blank and nearby pillars keep counting cells that can never hold their torches, so the counts never force the next one.
Where to look: The moment any torch lands, before anything else: its full row and column, both directions, up to the pillars.
Worked example
A real position from a board that needs Torches can't see each other. Purple: what the reasoning rests on. Dashed: the cells in question. Green: what it proves.
Halfway there. Next: the endgame habit, hunting the last dark room. Step 6: The last dark room →