Bridges: the only place threads may cross
Normally threads never cross. A bridge is the exception: one thread runs over it, the other under it, at a right angle. No turning on a bridge, and both passes must be filled.
Those three short rules make a bridge one of the most informative cells on the board. Let’s unpack what they force.
What a bridge decides
- Both passes are used. One thread crosses the bridge left to right, another top to bottom. So all four neighbours of the bridge are taken by these two threads (or by their buttons).
- No turning. The thread that enters from the left leaves to the right. The bridge is a straight piece in both directions.
- Four neighbours, two pairs. The cells to the left and right of the bridge belong to one thread. The cells above and below belong to the other.
That last point is the key. As soon as you know the colour of one neighbour, you know the colour of the opposite one. If a button sits next to the bridge, its thread goes straight across, and the cell on the far side is the same colour.
Reading a bridge level
Start with the neighbours of each bridge. Ask: which thread can reach both the left and the right side? Which can reach both top and bottom? Often only one candidate fits each direction, especially when buttons sit close to the bridge.
Then use the bridge as a gate. A thread that has to cross the bridge vertically splits the board into a left part and a right part everywhere except at the bridge itself. The horizontal thread is the only one allowed to get from one side to the other there. This kind of splitting is exactly what solves big boards (see /guides/big-boards/).
Common mistakes
- Leaving one pass empty. A bridge with a single thread isn’t finished, even if every button is connected.
- Turning on the bridge. That isn’t allowed — plan the turn one cell before or after it.
- Forgetting the edge rules. Corners and edges still give the first forced moves; the bridge comes second.
Two buttons of different colours next to a bridge on adjacent sides — say, left and top — already tell you both threads: one runs along the bridge, the other across it.
With two bridges on one board, look at them together. If the same thread could cross both, check whether it would cut off a button of another pair. Usually one bridge settles the other.
Practise in the Bridges set: ten levels from 5×5 with one bridge to 7×7 with two. The small and mean set has bridges on tight 6×6 boards — there every cell around the bridge matters.