Light Up
Place bulbs to light every square without lighting another bulb
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Click or tap an open square to toggle a bulb. Mark mode or right-click toggles a no-bulb mark. Arrow keys move; Enter or Space uses the selected mode; B toggles a bulb, I or Shift Enter toggles a mark, Backspace, Delete or 0 erases. Ctrl or ⌘ Z undoes; Ctrl or ⌘ Shift Z redoes.
Lit squares: – · Bulbs: –
Progress saves on this device.
How to play
Place bulbs to light every open square. A bulb lights its own square and its row and column until a wall blocks the light. Bulbs must never shine on each other.
A numbered wall needs exactly that many bulbs in the four neighbouring squares above, below, left and right. An unnumbered wall has no count requirement. Lit squares have a warm background; bulbs and impossible clue counts show ! when they conflict.
Use × marks for squares where you think no bulb belongs. Remove a mark before placing a bulb there; bulbs also block marks. Erase clears either. Undo and Redo follow your history; a new move after Undo replaces the old branch. Restart resets the same puzzle and can be undone. Lit squares have a corner dot; unlit squares have a dark fill.
Solve reveals an upstream solution and does not count as a win. Undo returns to the previous state. Some Hard puzzles may require guesses.
Puzzle by Nikoli (Akari). Engine: Simon Tatham’s Light Up, contributed by James Harvey. MIT license and credits.