Eight Corner Horse in Xiangqi: The 八角马 Checkmate

The name and its relatives

八 is eight and 角 is corner or angle. The same two characters begin 八角形, the Chinese word for an octagon, and xqinenglish translates the pattern as the Octagonal Horse. Chinese descriptions agree on the idea: a Horse on any of the four palace corners forms a diagonal with the enemy General and takes away its freedom to move, after which other pieces finish the job. Two other names make the purpose plain. 定将马 is the horse that fixes the General in place, and some lists call the mate 八角定将.

Some writers also use the name more loosely, for any Horse standing on one of the back corners of the enemy palace. Keep both meanings in mind when you read diagrams from different sources.

Why one Horse can freeze a General

A General on a palace corner is already short of room. It moves one point along the palace lines and never diagonally, so from a corner it has just two possible moves: one along the rank and one along the file.

Now put a Horse on the opposite end of the diagonal that runs through the palace centre. Each of the General's two neighbours is exactly one Horse leap away from it. The Horse cannot attack the General itself from there, and it doesn't need to. It has taken both escape points, so any check the General cannot block or answer by capture is mate.

There is a catch, the usual one with Horses. Each leap needs its leg point to be empty, and here the legs are the middle points of the two palace edges that meet at the Horse's corner. If either one is occupied, by a piece of either colour, one escape route opens again.

From a palcorner check to a frozen General

Set up this position, with Red at the bottom and left and right read from Red's side. Black's General is on its home point and one Black Advisor stands on the back corner on Red's left. Red has a Horse on Black's river bank rank, on the third file from the left edge, a Chariot on the right edge of Black's second rank, and its General on the left back corner of its own palace. Red to move.

There is no mate in one, so the Horse jumps to the front-left corner of Black's palace and checks the General on its home point. That is a palcorner check, and the engine shows Black has a single legal reply. The palace centre lies on the Chariot's rank and the left back corner holds Black's own Advisor, so the General must step right into the other back corner.

Look at what has happened. The Horse sits on the front-left corner and the General on the back-right corner, the two ends of one palace diagonal. The Horse covers the General's home point and the middle of the palace's right edge, so the General cannot move at all. Red's Chariot now slides along the second rank to that right-edge point, directly in front of the General, where the Horse guards it. Checkmate. Running the Chariot up the edge file to the corner of the board and checking along the back rank also mates, and the engine confirms both.

The order matters. The first check is not the mate. Its purpose is to push the General onto exactly the corner where the Horse can freeze it.

The back-rank version

The same geometry works the other way round, with the Horse on a back corner and the General stranded on a front corner. Put Black's General on the front-right corner of its palace and a Black Advisor on the back-right corner. Red has a Horse on Black's back-left corner, a Chariot on its own side of the river, on the rank next to the river and the second file from the right edge, an Advisor in the centre of its own palace, and its General at home. The Red Advisor is only there to close the central file, so the Horse gets the credit it deserves.

The Chariot slides two points left onto the rib file and checks straight up it. The General's two neighbours are the middle of the palace's front edge and the middle of its right edge, and the Horse covers both. Moving the Chariot up its own file to Black's third rank and checking along that rank works just as well. Take the Horse off the board, though, and the rib-file check is answered by the General stepping to the middle of the front edge.

How the defender breaks the diagonal

Start with the legs. A Horse on the front-left corner needs the middle of the palace's left edge and the middle of its front edge to be empty. Advisors can stand on neither point, and Elephants can never reach the side-edge point, but an Elephant can stand on the middle of the front edge, and an Elephant there takes half the net away for free.

Next, be wary of the corners. In the first example the General had no choice, but in real games it often does. When an enemy Horse is near your palace, treat the corner diagonally opposite it as poisoned and look for another point to run to.

Finally, remember that the corner Horse never touches the palace centre. Both examples above start with that point empty. When an Advisor stands there, the General's neighbours and the possible checks change, and the attacker has to count everything again.

Errors that let the General slip away

Attackers most often forget the legs. They see the diagonal, play the check, and find that their own Cannon or Soldier on the middle of the palace's front edge has switched off half of the Horse's control. The second error is checking from the wrong corner, which drives the General toward the centre instead of into the far corner. Before each check, ask where the General will go, and steer it onto the diagonal.

A smaller point: the two corners of the same palace edge are not interchangeable. Each one sits on a different diagonal, so each freezes the General on a different corner. Picture the diagonal first, then choose the corner.

Drilling the diagonal

Set up both positions, then move the Horse to each of the other three corners and work out where the General would have to stand to be frozen. After that, XiangqiPlay's 5,000 engine-proven mate puzzles will show you the eight corner horse hiding inside longer combinations.