How does the flying general rule change when a cannon also sits on the central file?
These answers restate the rules implemented by our open-source engine, which follows the WXF laws and is regression-tested against the official 110-diagram repetition casebook. Where an answer refers to a specific position, that position lives on the linked reference page, where the build verifies it against the engine before publishing.
Direct answer
The rule itself does not change, the two generals still may never face each other on a clear file. What changes is the stakes of clearing that file: with a cannon also sitting on it, removing pieces can create a cannon check and a flying general violation at the same time, so a capture sequence that looks routine can turn out to be partly illegal, or suddenly decisive.
Why this needs extra care to calculate
Normally you check a capture sequence for whether it wins material. With a cannon and both generals sharing a file, you also have to check whether any intermediate step would leave the file with exactly zero pieces between the generals, which is illegal regardless of whether the cannon's own check would otherwise have worked.
Where this comes up in practice
Most often after heavy trading down the central file in a middlegame that started with a Central Cannon. By the time only a couple of pieces remain on the file, both the flying general rule and any cannon on that same line are live factors in the same calculation, not separate concerns.
Related questions
Can a cannon's own screen also be the piece keeping the generals apart?
Yes, the same piece can be doing both jobs at once, which is exactly what makes clearing the file so consequential.
Does this only apply to the central file?
Yes, the flying general rule only ever concerns whichever file the two generals happen to occupy.
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