What is a double cannon mate (重炮) and how does it work?
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
重炮 (the double cannon mate) stacks two friendly cannons on the same file, one directly behind the other, using the front cannon as the rear one's screen. Because a cannon needs exactly one screen to capture, the front cannon can fire down the file whenever needed, and the defender usually cannot remove both cannons or block the file without losing material or leaving the general in check anyway.
Why it is so hard to defend once set up
A single cannon aimed at the general is normally something a defender can block with any piece. With two cannons stacked, interposing a piece to block the front cannon simply becomes the new screen the rear cannon needed all along, so a blocking piece can make the position worse rather than better. See the double cannon mate for the position-by-position mechanics.
Getting the setup in the first place
The harder part is usually not the mate itself but manoeuvring two cannons onto the same open file against a defender who can see it coming, which typically requires other threats to distract the defence first. It is rarely the opening move of a combination; it is usually the point everything else was building toward.
Related questions
Can the defender ever safely block a double cannon setup?
Occasionally, if the blocking piece is itself defended well enough to survive being captured by the front cannon, but this is the exception rather than the rule.
Does this require the file to already be fully open?
Yes, both cannons need a clear path to the general along that file for the mate to actually land.
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