What is a river-patrolling cannon (巡河炮)?
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
A 巡河炮 (river-patrolling cannon) is a cannon advanced two points to its own river bank, for example 炮八進二 (C8+2). From there it controls the whole river-bank rank sideways, so it can shift across the board in one move to defend a soldier, block, or open a new line of attack.
What the patrol gives
On its starting rank the cannon's sideways movement is cut short by the horses and other pieces. On the river bank the rank is usually more open, so the cannon can travel along it. It can support its own soldiers, meet an enemy soldier or piece arriving at the river, and move to a new file when a target appears. It stays on its own side, so it is less exposed than a cannon that has crossed.
The same idea with a chariot
A chariot can patrol the river too. In Central Cannon lines, once a chariot has come to file 2, 車二進四 (R2+4) is the typical 巡河車: it puts the chariot on the river-bank rank, where it covers the rank from edge to edge and watches enemy soldiers trying to cross. The chariot does it more forcefully, since it attacks along the rank directly while a cannon needs a screen to capture.
Its limits
A patrolling cannon is flexible but not threatening in itself; it does not attack the enemy palace from there. It can also be harassed by enemy soldiers or horses arriving at the river. Treat it as a positional choice: useful when you want flexibility and control of the middle, less so when you need direct pressure. See using cannons in the opening.
Related questions
Is a river-patrolling cannon the same as crossing the river?
No. It stays on its own river bank. A cannon that goes across is a different placement with different risks.
Does this placement have a name in notation?
No special symbol; it is written as an ordinary advance, such as C8+2.
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