Fishing the Moon in Xiangqi: The 海底捞月 Checkmate

An idiom about the impossible

海底捞月 is an everyday Chinese idiom: to scoop the moon up from the bottom of the sea. It describes a hopeless effort, like trying to catch the moon's reflection in water. On the board the joke is turned around. The sea floor is the defender's back rank, the edge of the board behind the palace, and the moon is the General, which the attacker really does fish out from underneath. Some books use the alternative name 沉底月 (chén dǐ yuè), the sunken moon.

Sources draw the edges of the pattern in different places, so it helps to know that up front. Some Chinese mate lists define it broadly, as any attack launched from the enemy's back rank behind the General, supported by your own General's grip on the centre. Others tie it to one endgame, Chariot and Cannon against a lone Chariot, where the Cannon slips behind the enemy General to drive the defending Chariot away. One well-known glossary adds that a Chariot and Soldier can do the same job. We cover the broad version first and the endgame technique after it.

Why the back door is open

The General can never leave its palace, which has only three files. When your own General stands on the central file with nothing between the two Generals, the enemy General may never step onto that file, because the Generals would face each other. That leaves it a single rib file, one of the palace's side files, with three points on it.

A defender in that situation guards the rib file from the front, usually with a Chariot standing further down the same file, so checks from below are blocked. What that Chariot cannot do is guard the point behind its own General, because the General itself stands in the way. That point on the back rank is the hole the whole pattern is built around.

Worked example: round the back

Set up the board with Red at the bottom, reading left and right from Red's side. Black's General has stepped up to the middle point of the left side of its palace. Black has one Advisor, on the back corner of the palace on Red's right, and a Chariot on its General's file, just across the river on Red's side, blocking any check from below. Red has its General on its home point with the central file empty, a Chariot in the corner of Black's back rank on Red's left, and a Cannon on Black's back rank on the right, on the point where Black's Horse starts the game.

The obvious check fails. If Red's Chariot drops one rank and checks along Black's second rank, the General simply steps forward to the front corner of its palace, and the engine lists that as Black's one reply.

The strong move looks quieter. Red slides the Chariot along the back rank until it stands directly behind Black's General. That is check from behind, and Black has nothing. The General cannot capture the Chariot, because Red's Cannon guards it by jumping over Black's own Advisor. It cannot step toward the centre, because the Generals would face each other. Stepping forward keeps it on the Chariot's file. Black's Chariot is on the right file but the wrong side of it: its own General blocks the path to the checking piece. Checkmate, confirmed by the site's rules engine.

The endgame version

The textbook setting is Chariot and Cannon against a lone Chariot, with the attacker's General on the central file. As Chinese endgame books describe the method, the Cannon goes behind the enemy General with the Chariot guarding it, uses that General as a screen to chase away the defending Chariot, and then the Chariot pulls back to finish with the facing Generals.

Here is the key moment. Black's General is again in the middle of its left rib file, with its Chariot further down that file. Red's Chariot stands on Black's back rank in the centre, on the point where Black's General started, and Red's Cannon has just landed on the back corner directly behind Black's General. Red's General is on its home point.

It is not check, but look at what the Cannon hits. Jumping over Black's General, it attacks Black's Chariot. The General cannot take the Cannon, because Red's Chariot next to it guards it. Moving the Chariot up or down the same file does not help, since every point it can reach on that file stays in the Cannon's line of fire. In practice the defending Chariot has to leave the file.

Once it has gone, the finish is a retreat. Red's Chariot drops back down the central file to Red's soldier rank, then steps across onto Black's General's file and checks from below. In the final position Black's General is still in the middle of its rib file with Red's Cannon behind it, Red's Chariot is on the same file on Red's soldier rank, Red's General is at home, and Black's Chariot is far away on the left edge of Red's side. The General cannot step into the centre because of the facing Generals, and capturing the Cannon behind it leaves it on the Chariot's file. The Cannon has been left hanging as bait, and it does not matter. Checkmate.

What the defender can do

Keep the General on its home point when you can. The pattern works because a raised General leaves an empty point behind it; a General on the back rank has nothing behind it at all.

Break the central bolt. The whole idea leans on the facing Generals, so a piece of yours on the central file between the two Generals changes everything. Put a Black Elephant on the middle point of the palace's front edge in the first example and Red's check from behind is no longer mate, because the General steps to the palace centre. The engine confirms that escape.

Watch your own pieces as possible screens. In the first example Black's Advisor was what let Red's Cannon guard the Chariot. Whenever an enemy Cannon reaches your back rank, ask which points it covers by jumping one of your pieces. And in the Chariot endgame, try to stop the Cannon settling behind your General in the first place, using checks and threats to gain time rather than parking your Chariot passively on the file.

Common slips

Checking from the front out of habit is the first. Against a General guarded from below, a check up the file or along the side usually just pushes it to a new point. Look behind it first.

Forgetting what guards the Chariot is the second. A Chariot checking from the point directly behind the General is adjacent to it, so it needs protection, from a Cannon jumping a screen, a Soldier beside it or some other piece. Count the Cannon's screen exactly: one piece, not zero and not two.

The third is blocking your own bolt. The facing Generals only help while the central file between the two Generals is empty, so parking your own Chariot or Cannon there switches the trick off.

Practising the back-rank eye

Attacks from behind are easy to miss because most of us are trained to look at the front of the palace. XiangqiPlay's free puzzle library, with 5,000 engine-proven mate puzzles, is a good place to retrain that habit, and a free account lets you test the endgame method against the computer at any of its eight levels, Easy to God.