Easy
Naked Single
Only one candidate height remains for this cell — place it.
Smart Hint: Skyscrapers and Skyscrapers+Hidden Single
A height has only one possible cell left in this row or column, even though that cell has other candidates too.
Smart Hint: Skyscrapers and Skyscrapers+Clue of One
A clue of 1 means the tallest building is visible right at that edge — place the max height there.
Smart Hint: Skyscrapers and Skyscrapers+Full Visibility Clue
A clue equal to the grid size means every building is taller than the last — the whole line is forced into one exact order.
Smart Hint: Skyscrapers and Skyscrapers+Medium
Visible-Count Bound
At each position, leave enough taller heights behind to reach the clue's visible count — eliminate every height too tall to allow that.
Smart Hint: Skyscrapers and Skyscrapers+Line Consistency (≤4 Blanks)
Every way to fill this row or column's remaining heights that matches its clue was checked — a height eliminated here doesn't appear in any of them. Enough of the line was already filled in that there was not much left to weigh.
Smart Hint: Skyscrapers and Skyscrapers+Naked Pair
Two cells in a row or column share the exact same two candidate heights — remove those heights from the rest of the line.
Smart Hint: SkyscrapersHard
Line Consistency Sparse (5+ Blanks)
The same line-ordering check on a line that is still mostly blank — little to anchor it, so nearly every arrangement the clue allows has to be weighed. Same idea as Line Consistency, a good deal more work.
Smart Hint: Skyscrapers and Skyscrapers+Hidden Pair
Two heights only fit in the same two cells of a row or column — strip those cells down to just those two.
Smart Hint: SkyscrapersNaked Triple
Three cells in a row or column whose candidates combine to exactly three heights — eliminate those heights elsewhere in the line.
Hidden Triple
Three heights only fit in the same three cells of a row or column — strip those cells down to just those three.
X-Wing
A height's only two candidate cells line up in the same two columns (or rows) across two rows (or columns) — eliminate it everywhere else in those columns or rows.
Smart Hint: Skyscrapers and Skyscrapers+Naked Quad
Four cells in a row or column whose candidates combine to exactly four heights — eliminate those heights elsewhere in the line.
Hidden Quad
Four heights only fit in the same four cells of a row or column — strip those cells down to just those four.
Expert
XY-Wing
A cell with two candidates links to two other two-candidate cells that share one candidate each with it and one with each other — the shared height can't be in any cell both of those see.
Smart Hint: Skyscrapers and Skyscrapers+W-Wing
Two cells share the same candidate pair and aren't in the same row or column, but a chain elsewhere forces one of them to one of those heights — eliminate the other height from any cell both see.
Smart Hint: Skyscrapers and Skyscrapers+XYZ-Wing
A three-candidate cell links to two two-candidate cells whose combined candidates match it exactly — the shared height can't be in any cell all three see.
Swordfish
A height's candidate cells across three rows (or columns) line up in the same three columns (or rows) — eliminate it everywhere else in those columns or rows.
Smart Hint: Skyscrapers+Cross-Line Consistency
This row or column has only a few arrangements left that match its clue — any one that leaves some other row or column with no way to finish is out, and a height used by none of the survivors can be struck.
Smart Hint: Skyscrapers and Skyscrapers+Extreme
Jellyfish
A height's candidate cells across four rows (or columns) line up in the same four columns (or rows) — eliminate it everywhere else in those columns or rows.
Smart Hint: Skyscrapers+Cross-Line Forcing
Same as Cross-Line Consistency, but each arrangement is also followed through: fill in every height it forces outright, and rule it out if that leaves some other row or column unfinishable.
Smart Hint: Skyscrapers and Skyscrapers+