Can two cannons share the same screen piece at once?
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
Yes. A screen is just a piece standing between a cannon and a target; nothing limits how many cannons can use it at once. A piece sitting between two opposing cannons on the same file, for example, is a legal screen for both of them at the same time, each threatening to capture in the opposite direction across it.
A standoff along one file
Picture a red cannon, an enemy piece, and a black cannon, in that order, all on the same open file. The piece in the middle is screening for both sides at once: the red cannon threatens to jump it and take the black cannon, and the black cannon simultaneously threatens to jump it and take the red one. Whoever captures first usually wins the exchange, since removing the screen also removes the other cannon's threat.
Why this matters for calculation
A single piece caught between two cannons is under far more pressure than its own value suggests, because it is effectively holding two separate threats in balance. Moving it, capturing it, or reinforcing it all change the position for both cannons at once, not just one.
Related questions
Does it matter whether the shared screen belongs to either side?
No. Colour is irrelevant to what counts as a screen; a piece of either side works for either cannon.
What happens if the screen piece is captured by something else entirely?
Both cannons lose that particular threat at once, since neither has a screen left on that line any more.
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