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Light Up: One Way to Light

A dark square with only one place left that could light it gets lit from there. That place might be along its row or column, or the square itself.

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

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

The rule

  1. Pick a square that’s still dark.
  2. List the places that could light it: the square itself, and the squares along its row and column up to the first black square, leaving out any with a dot.
  3. If only one is left, the bulb goes there.

In a real puzzle

On the 12×12, it’s first needed at step 24. Row 2, column 11 is still dark. The 0 nearby has ruled out its left neighbour, a black square stands to its right, and every square above and below it in its column is already ruled out. Nothing else can light it, so it needs a bulb of its own.

Hard level 4, step 24 one-way-to-light: bulb row 2, column 11 (from row 2, 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 24: row 2, column 11 can only light itself.

How often you’ll need it

On 48 of the 50 Easy levels, 49 Medium and all 50 Hard: 3.5 times per Easy level on average, 6.4 on Medium and 10 on Hard. It’s 7 of the 42 steps on the 12×12 here.

Next steps

Questions

Can a square light itself?

Yes. A bulb lights its own square. A dark square boxed in by black squares and dots often needs a bulb of its own.