TECHNIQUE 1 OF 4
Zero lines: eliminate without placing a tent
A zero clue gives certainty immediately. It can also remove candidates for trees in neighbouring lines, making a later count or adjacency deduction possible.
The rule
A row or column with clue 0 contains no tents. Mark its non-tree squares as grass. Leave the trees in place: a tree in a zero row can have its tent immediately above or below it, provided the other constraints permit that location.
A common mistake
Treating a zero line as containing no trees changes the puzzle. Another mistake is stopping after marking grass without reconsidering the candidate squares of nearby trees.
TRY IT 1 · tents-zero-lines-1
| C11 | C21 | C30 | C41 | C51 | C62 | |
|---|---|---|---|---|---|---|
| R11 | ||||||
| R21 | ||||||
| R30 | ||||||
| R42 | ||||||
| R50 | ||||||
| R62 |
What can you justify?
Use the zero lines technique. What does it prove about R2C3? Treat the displayed starting marks as established facts.
Written explanation
A row or column clue of zero allows no tents. Mark R2C3, R3C1, R3C2, R3C3, R3C4, R3C6, R4C3, R5C1, R5C2, R5C3, R5C5, R6C3 as grass; tree squares stay unchanged.
TRY IT 2 · tents-zero-lines-2
| C12 | C21 | C32 | C41 | C51 | C60 | |
|---|---|---|---|---|---|---|
| R12 | ||||||
| R21 | ||||||
| R31 | ||||||
| R41 | ||||||
| R50 | ||||||
| R62 |
What can you justify?
Use the zero lines technique. What does it prove about R1C6? Treat the displayed starting marks as established facts.
Written explanation
A row or column clue of zero allows no tents. Mark R1C6, R2C6, R3C6, R4C6, R5C1, R5C3, R5C4, R5C5, R5C6, R6C6 as grass; tree squares stay unchanged.
Zero tents does not mean zero trees
Row 3 has clue 0 and contains a tree at R3C3. Its neighbouring squares R3C2, R3C4, R2C3 and R4C3 have not otherwise been classified.
The tempting move
Remove or mark over the tree because its row is supposed to contain zero.
Why it fails
The row clue counts tents, not trees. The tree remains part of the puzzle. Its tent cannot be at R3C2 or R3C4, but could be above or below it if the other constraints allow.
What is actually justified
Keep the tree. Mark the row's non-tree squares as grass, then examine R2C3 and R4C3 against their other constraints. This information alone does not choose between them.
Transfer the idea
A tree lies in a row with clue 0. Its left and right neighbours are ordinary squares. Can either be its tent? Where must you look instead?
Check your reasoning
Neither side can contain a tent because both squares belong to the zero row. Look directly above and below the tree, where those squares exist, then check their column counts and tent spacing. A diagonal square is never a valid substitute.