TECHNIQUE 3 OF 4
Crossings: one connection can remove another
A newly established bridge affects more than its two islands. Any possible route that crosses it disappears, potentially forcing a decision elsewhere.
The rule
Horizontal and vertical bridges cannot cross between islands. Once at least one bridge is established along a route, any crossing candidate route must carry zero. Connections meeting at the same island are not crossings; they are how the network joins. A second parallel bridge uses the same route and does not create additional crossing permissions.
A common mistake
Drawing a candidate as though it were a placed bridge can make the board look contradictory. A crossing exclusion must follow from a connection you have established, not one you merely prefer.
TRY IT 1 · bridges-crossing-1
What can you justify?
What range can the displayed crossing deduction establish for route F–B?
Written explanation
C–D is established with at least one bridge. F–B cross that route between islands, so they must carry zero bridges.
TRY IT 2 · bridges-crossing-2
What can you justify?
What range can the displayed crossing deduction establish for route H–E?
Written explanation
A–B is established with at least one bridge. H–E cross that route between islands, so they must carry zero bridges.
A junction at an island is not a crossing
An island is at R3C3. A placed bridge joins it to an island at R3C1. Another candidate route joins R3C3 to an island at R1C3, with no intervening island.
The tempting move
Rule out the upward route because it meets the horizontal bridge at a right angle.
Why it fails
The routes meet at their shared endpoint, R3C3. That is an island junction, which the rules allow. Forbidden crossings occur between endpoints without an island at the intersection.
What is actually justified
The upward route remains geometrically legal. Its endpoints' remaining counts and the rest of the puzzle determine whether it is used.
Transfer the idea
A horizontal route from R3C1 to R3C5 contains one established bridge. Can a vertical route from R1C3 to R5C3 be used? Would the answer change if the horizontal connection were double?
Check your reasoning
No. The routes cross at R3C3, between their endpoints. A single established bridge already forbids that crossing, and making it double does not change the restriction. This example assumes there is no intervening island on either candidate route.