What is a hollow cannon (空頭炮)?
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
空頭炮 — "hollow-head cannon" — is a cannon on the enemy general's file with nothing at all between them. It gives no check, because a cannon needs a screen. What it does is far worse for the defender: the general can never move onto that file, and any piece that steps onto the line becomes a screen and is instantly attacked.
Why it is so unpleasant to face
It freezes a whole file. The defender cannot interpose anything without handing over a target, cannot let the general use the file, and often cannot remove the cannon. Many attacks are built by establishing 空頭炮 first and then bringing a second piece.
Getting rid of one
The usual answers are to capture the cannon, to block the file with a piece that is defended so being a screen costs nothing, or to move the general off the file if it can. The one thing that does not work is blocking with an undefended piece.
Related questions
Is a hollow cannon giving check?
No. With an empty line there is no capture available, therefore no check.
Is it stronger than a cannon with a screen?
Different. A screened cannon threatens now; a hollow cannon paralyses the file and threatens everything that later steps onto it.
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