Plankway

Crossing bridges: one certain bridge blocks another

Bridges may not cross each other. That rule is easy to remember while you draw, but it is also a solving technique: a certain bridge is a wall that other bridges cannot pass.

The idea

Whenever a bridge is certain, look along it. Is there a connection between two other islands that would have to cross it? Then that connection is impossible, even though both islands can see each other.

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The bridge from C1 to C5 is certain. The connection from A3 to E3 would cross it, so it is ruled out.

What it leads to

In this example, the two 2s in A3 and E3 each just lost their sideways neighbour. Each now has only one neighbour left, below it, and needs two bridges from it: double bridges.

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Without the crossing connection, both 2s take a double bridge downwards.

From there, the 3s in the bottom row fill up with single bridges to the 3 in C5. The finished puzzle:

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The solution of this example.

Spotting it

  • Long bridges through the middle of the board block the most. After placing one, scan the islands on both sides of it.
  • Crossings also work the other way: if two long connections would cross and one of them is certain, the other is not. If neither is certain yet, the islands around them often decide which one it is.
  • The board in Plankway never lets you build a crossing bridge, so you will not make this mistake by accident. The point is to use the rule to rule things out before you try.

Recent puzzles that needed this technique

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