Tents and Trees

Tents and Trees (also called Tents, or Zeltlager in German) is a logic puzzle played on a grid containing trees. The player must place one tent next to each tree (horizontally or vertically adjacent) so that no two tents touch each other, even diagonally. Row and column numbers at the edges tell the player exactly how many tents are in each line. The puzzle combines spatial reasoning with counting constraints to create an engaging challenge.

Rules

1. The grid contains some cells with trees. All other cells are empty.
2. Place exactly one tent for each tree. Each tent must be orthogonally adjacent (up, down, left, right) to its paired tree.
3. Each tree is paired with exactly one tent, and each tent is paired with exactly one tree.
4. No two tents may be adjacent to each other, not even diagonally (tents cannot touch in any of the 8 surrounding cells).
5. Numbers along the edges of the grid indicate exactly how many tents are in that row or column.
6. All remaining cells (not trees, not tents) are grass (empty).

Strategies

• Zero Rows/Columns: If a row or column has a count of 0, all non-tree cells in that line can be marked as grass.
• Full Rows/Columns: If the number of available cells in a row or column equals the tent count, all available cells must contain tents.
• Isolated Trees: If a tree has only one possible adjacent empty cell (the others are trees, edges, or already occupied), its tent must go there.
• Tent Exclusion Zones: Once a tent is placed, all 8 surrounding cells are marked as grass (no other tent can touch it). This propagates constraints significantly.
• Counting: If a row or column already has all its tents placed (count reached), all remaining empty cells in that line are grass.
• Tree-Tent Pairing: Sometimes even if you cannot determine the exact tent position, you can determine which tree a tent must pair with, which limits other trees' options.

Tents and Trees was created by the Dutch puzzle designer Leon Balmaekers. The puzzle first appeared in the early 2000s in puzzle magazines and quickly gained popularity through online puzzle communities.

The puzzle has roots in constraint-satisfaction problems studied in computer science, but its presentation as a camping-themed puzzle gives it an accessible, friendly character. The idea of pitching tents next to trees in a campground, with the constraint that tents need space between them, provides an intuitive metaphor that makes the rules easy to remember.

Tents and Trees gained wider recognition through its inclusion in the World Puzzle Championship and various national puzzle competitions. The puzzle's combination of counting (row/column numbers) and spatial reasoning (adjacency constraints) makes it appeal to solvers who enjoy puzzles that require multiple types of logical thinking simultaneously.

The puzzle has become a staple of online puzzle platforms, including puzzle-tents.com and Simon Tatham's Portable Puzzle Collection (under the name "Tents"). Its clear visual presentation and satisfying solving process have made it one of the most popular modern grid puzzles. Puzzle designers particularly enjoy constructing Tents puzzles because the dual constraint system (pairing + non-adjacency) allows for elegant puzzles with unique solutions even on small grids.