Heaven and Earth Cannons in Xiangqi: The 天地炮 Mate

Why heaven and why earth

The names describe where each Cannon stands. The heaven cannon (天炮) is the Cannon at the head of the attack, the familiar central cannon (当头炮, dāng tóu pào) aimed down the middle file. The earth cannon (地炮) is a sunk cannon (沉底炮, chén dǐ pào), one that has travelled all the way to the defender's bottom rank and lies along the ground behind the palace. One strikes from the front, the other from underneath, and the palace is caught between sky and floor.

Chinese glossaries describe each Cannon's job quite precisely: the central one ties down the middle Advisor and middle Elephant, and the bottom one ties down the Advisor or Elephant on the back rank. The mate is often grouped with the Iron Bolt (铁门栓), which also starts from a central Cannon, and it shares one condition with the double cannon mate: neither Cannon may be capturable.

Pins do the heavy lifting

Most mates need one checking piece and some others to cover escape points. The unusual thing here is that the covering is done by pins. A Cannon behind two enemy pieces gives no check, but if either piece moves away the other becomes a lone screen, and the Cannon checks at once. Since a move that leaves your own General in check is illegal, neither piece may move.

So the heaven cannon, sitting behind an Advisor on the palace centre and an Elephant in front of it, attacks nothing at all. It simply glues both pieces to the file. The earth cannon then gives the actual check along the back rank, using a corner Advisor as its screen. The defender looks at a palace full of pieces and finds that none of them can move.

A position to set up

Put Red at the bottom and read left and right from Red's side. Black's General is on its home point. One Black Advisor stands on the palace centre, the other on the back corner of the palace on Red's left. One Black Elephant stands on the middle point of the palace's front edge, directly in front of that central Advisor, and the other is on its starting point on Red's right. Red has a Cannon on the central file on its own side of the river, its other Cannon on its left-hand starting point, and its General on its home point.

Nothing is in check yet. The heaven cannon has two Black pieces in front of it, the Elephant and the Advisor, so it stays silent.

Red now sends the second Cannon straight up its file to Black's back rank, landing on the point where Black's left Horse began the game. From there it looks along the rank: one empty point, then the corner Advisor as a screen, then the General. Check.

Black has no answer. The General cannot step forward, because its own Advisor occupies the palace centre, and stepping right along the back rank keeps it on the earth cannon's line. The corner Advisor is stuck, since its only move is to the occupied centre. The central Advisor and the front Elephant are both pinned by the heaven cannon; the Elephant could otherwise drop back and block the back rank, but moving it would leave the Advisor as a lone screen. Nothing can reach either Cannon. Checkmate, and the site's rules engine agrees.

Building it in a real game

The heaven cannon is often in place from the opening, since the Central Cannon is one of the most popular first moves in the game. The real work is the earth cannon's route. A Cannon that reaches the back rank needs an open file to travel along, which usually means the defender's Horse has left its starting point and the Soldiers on that file are gone.

Count the screens on both lines before committing. The central file wants exactly two defending pieces between the heaven cannon and the General, so that both are pinned. With one, it is simply a check the defender can deal with; with three, nothing is pinned. The back rank wants exactly one screen between the earth cannon and the General.

A Chariot is the usual third partner. In many real finishes the two Cannons hold everything in place while a Chariot runs down a rib file or along a palace rank to close the net. Think of the pair of Cannons as the net and the Chariot as the hand that draws it shut.

Keeping the palace breathing

Defending starts with not letting both lines lock at the same time. Two pieces on the central file are only dangerous if nothing else stands there to break the pin. Go back to the example and return Black's central Soldier to its starting point in front of the Elephant. Now the heaven cannon has three pieces ahead of it and pins nothing. The earth cannon still gives check, but the front Elephant can drop back to the back rank and block, and the engine lists that as Black's answer. One extra piece turns mate into an ordinary check.

The back rank can be protected in the same way. If Black's other Elephant were on its starting point on Red's left, beside the corner Advisor, the earth cannon would have two pieces to jump and would give no check at all.

Beyond that, challenge a sunk cannon early with a Chariot or Horse before its partner arrives, think twice before trading off the pieces that keep your central file crowded, and whenever an enemy Cannon lands on your back rank, ask straight away which of your pieces it has just pinned.

Errors that let the defender off

Landing the earth cannon where it can be taken is the classic mistake. The whole mate depends on both Cannons staying on the board, so a defender who can capture either one is off the hook.

Miscounting the central file is almost as common, usually by forgetting that your own central Soldier is still standing there. And playing for the mate while the defender has a spare piece near the palace rarely works, because a single free piece is often all it takes to block. Remove or distract that piece first.

Where to practise

Seeing both lines at once takes repetition. Work through the mate puzzles in XiangqiPlay's free library of 5,000 engine-proven positions, then try steering toward the pattern against the computer, which you can play at any of its eight levels (Easy to God) with a free account.