New: Kerb Clash, clear the jammed car parkNew: Sudoku and Minesweeper, classic puzzles that play offlineInstall any game to your home screen

Light Up: Leaving a Square Dark

Every dark square needs some way to be lit. A square whose bulb would rule out every way left to light another square gets no bulb.

Open Light Up full screen for more sizes, saved progress and best times.

This is the last of the Light Up techniques. Each page shows its technique on the same 12×12 from the game, Hard level 4.

The rule

  1. Find a dark square with only a few ways left to be lit.
  2. Look for a square that would light all of them, usually where its row and column cross.
  3. A bulb there would leave the dark square with no way to be lit: put a dot on it.

In a real puzzle

On the 12×12, it’s first needed at step 40, near the end. Row 12, column 11, beside the 0, can’t have a bulb of its own, and only two squares can still light it: the one above it and the one to its right. A bulb in row 11, column 12 would light both, leaving it dark. So no bulb there, and the last two steps put bulbs in those two squares.

Hard level 4, step 40 leaves-dark: no bulb on 1 (from row 11, column 12 & row 12, column 11) 2 1 0 1 2 1 0 1 2 0 1 1 1 1 4 0 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12
Step 40: a bulb here would leave row 12, column 11 dark.

How often you’ll need it

Never on Easy, on 13 of the 50 Medium levels and 27 Hard: 0.3 times per Medium level on average and 0.7 on Hard. It’s 1 of the 42 steps on the 12×12 here.

Next steps

Questions

How do I spot it?

Look for a dark square with only two ways left to be lit, one along its row and one along its column. The square where those two lines cross, diagonal to it, would light both.