What is 天地炮, the 'heaven and earth cannons' pattern?
These answers restate the rules implemented by our open-source engine, which follows the WXF laws for movement, checkmate and both repetition rulings: a side that forces a repetition by perpetual check or perpetual chase loses rather than drawing. The chase half has not been validated against the official casebook. Where an answer refers to a specific position, that position lives on the linked reference page, where the build verifies it before publishing.
Direct answer
天地炮 ("heaven and earth cannons") is an attack with two cannons on two different lines: one on the central file, aimed at the general through the pieces in front of it, and one on the enemy's back rank, aimed along it. Together they press the general from the file and from the rank at once, and one more attacker, such as a chariot or horse, is often enough to decide the game.
Why attacking from two directions is the point
A defence set up to deal with one cannon's line often has nothing spare to cover a second cannon threatening from a completely different angle. Because the two cannons are placed so differently, the defender frequently cannot use the same piece to answer both, which is exactly the kind of overload described more generally in overloading a defender.
Getting the pieces into position
Getting one cannon to the central file and the other down to the enemy's back rank usually takes real preparation, since it means committing one cannon early while keeping the other in reserve until the right moment. It is rarely the opening move of a plan; more often it is the shape a longer build-up is aiming for once both cannons find their squares.
Related questions
Is this related to the double cannon mate?
Related in spirit but different in structure: 重炮 stacks two cannons on the same line, while 天地炮 deliberately uses two different lines.
Does this require the file to be fully open?
Not necessarily for both cannons at once; the two threats can develop on separate schedules rather than needing to land simultaneously.
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