List every ordering of a row or column that still matches its clues and notes. A height no ordering puts in a square is ruled out there; a height every ordering puts there is placed. It’s needed on every level in the game.
This is one of the Skyscrapers techniques. Each page shows its technique on the same 7×7 from the game, Hard level 1.
The rule
- Pick a line with clues and only a few heights left to place.
- List the orderings of its empty squares that its notes allow.
- Keep only those that show the right number of buildings from each clue.
- Mark what every ordering left agrees on, and cross out what none of them uses.
In a real puzzle
On the 7×7, it’s first needed at step 6. The orderings that the clues of row 2 and of column 1 still allow, with the notes so far, leave row 2, column 1 only one height: 5.
How often you’ll need it
On every one of the game’s 150 levels: 5.2 times per Easy level on average, 9.1 on Medium and 11.5 on Hard. It’s 6 of the 47 moves on the 7×7 here.