Plankway

Bridge limits: when a double bridge does not fit

A connection allows up to two bridges, but the islands on both ends have the final say. If an island only has room for one more bridge, none of its connections can become a double bridge.

A 1 never gets a double bridge

The simplest case: an island with a 1 can only take one bridge in total. So whatever it connects to, it is a single bridge.

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The 1 in A1 gets at most one bridge on each side; the dashed lines are still possible, a double is not.

This does not tell you yet where the bridge goes. But combined with the other techniques it often does: the 4 in D1, for example, can now count on at most one bridge from the 1, so it needs at least three from elsewhere.

Remaining room

The same reasoning works for any island that is partly finished. Subtract the certain bridges from its number: that is its remaining room. No single connection can get more than that.

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The 3 in C2 already has two bridges. It has room for one more, so neither the bridge to the right nor the one down can be double.

Why this matters

Limits mostly help other islands. Every time a connection is capped at one, its neighbour’s maximum goes down, and a lower maximum can trigger every bridge counts or at least one on that neighbour. Plankway’s solver uses bridge limits in almost every puzzle, and in the archive you will see it among the most used techniques of every day.

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