In the summer of 1851, while London crowded into the Crystal Palace to see the Great Exhibition, the strongest chess players in the world were in the same city for the first international tournament ever held. Between rounds, two of them sat down to a friendly game. Adolf Anderssen, a mathematics teacher from Breslau who would win the tournament, played White. Lionel Kieseritzky, a professional who made his living teaching chess at the Café de la Régence in Paris, played Black.

Anderssen gave away a bishop, then both his rooks, then his queen, and mated with the three small pieces he had left. A few years later the Austrian master Ernst Falkbeer called it "the Immortal Game", and the name stuck.

The tournament

The London tournament of 1851 was organised by Howard Staunton, the leading English player, to coincide with the Great Exhibition. It was a knockout, played over several weeks, and it is generally counted as the first international chess tournament. Anderssen won it, beating Staunton along the way, and was regarded for the following years as the strongest player in the world. There was no formal title of world champion yet, so that is a judgement rather than a record.

Casual games between the competitors were common, and they were played for pleasure, often quickly. That matters for how we read this one. Kieseritzky was not defending as if his reputation depended on it. He was playing a sociable game with a friend, and he took what was offered.

Who was playing

Anderssen taught mathematics at a school in Breslau, then in Prussia, and played chess in his spare time. He was not a professional, and by most accounts he was a modest man. He went on playing at the top level for decades.

Kieseritzky was the professional of the two. He had grown up on the Baltic coast, settled in Paris, and earned his living there from lessons and games at the Café de la Régence, the same café where Légal had played a century before. He was a fine attacking player himself, and an inventive opening theorist. He died only two years after this game. It is a little unfair that he is best remembered for losing it, but he would probably have understood: he published the game himself, in the French chess magazine La Régence, within weeks of playing it. It is partly thanks to the loser that we have it at all, which says something good about him.

The game in four movements

The chase. Black's queen comes out early and goes for a long walk. White does not chase her with pieces. White pushes pawns, one after another, each taking away a square, until the most powerful piece on the board has nowhere comfortable to stand.

The gifts. White lets a bishop be taken rather than lose time saving it. Then White offers a rook, and then the other rook. Black, playing for fun, takes everything. By the twentieth move Black is ahead by more than a rook.

The wall. One quiet pawn move, 19.e5, does more than any capture. It stands across the long diagonal between the black queen in the far corner and her king. She can no longer come home.

The errand. White gives the queen with check. The only black piece that can take her is the knight guarding the square next to the king. It takes, leaves its post, and White's bishop steps into the gap. Mate, with Black's rooks and one bishop still on the squares where they started the game.

What is known, disputed and legendary

• Known. The players, the year and the city, and that it was a casual game. The moves as they have been published since.
• Disputed. The ending. Kieseritzky himself indicated the game stopped before mate, so the famous finish may have been worked out afterwards. Versions also disagree about the order of some moves. And analysts since Steinitz have argued that Black's eighteenth move was not the best.
• Legend. That this game, more than any other, proved that attack beats material. It shows what attack can do against a careless defence. It does not prove much more than that.

Why it is still taught

The Immortal Game is the clearest demonstration in chess that material is only worth something if it can take part. A rook in the corner that never moves is, for the purposes of the next few moves, not on the board at all. Teachers use it to break children of a habit that is very natural: counting what they own rather than what they can use.

That is a lesson with a long reach. A team with more players is not stronger if half of them are in the wrong place. A budget is not bigger because money is tied up where it cannot be spent. Children grasp this very quickly when it is shown on a board, because the board makes the distance visible. You can point at the black rook on h8 and ask how many moves it would need to help. The answer, at the end, is that it cannot help at all: the king is in its way, and there is no time. A grown-up who has spent a career learning that resources in the wrong place are not really resources will recognise the lesson at once.

It is also worth teaching the doubts alongside the game. A child who learns that the most famous attacking game in history may have a composed ending, and was probably not forced, learns something about how stories improve in the retelling. They can still love the game. They will just hold it a little more carefully.

A note on the name

"Immortal" was a compliment from another player, given a few years after the event, not something either man said at the board. It is worth noticing how often a story's most famous label is added later by someone who was not there. The label then shapes how everyone afterwards sees the original. If you tell a child the game is immortal before you show it, they will look for brilliance. If you show it first and let them name it, they may notice something else, such as the black rooks that never moved, which is just as instructive.

The question for the table

If you play it through with a child, keep two tallies on a scrap of paper: what each side has, and what each side has near the enemy king. Stop after Black's twentieth move and compare them. Ask: "Who is winning?" Then ask: "How can both columns be right?"

Your child may have just read the same game as The Immortal Game at the Slow Lake Academy, where a badger named Warden Snare closes the queen's gates, a mole named Mason builds a wall across her road home, and a stoat named Sir Sidetrack sends the last guard on an errand.

A word about the doubts

You might wonder whether it is wise to tell a child that the most famous game in chess may not have ended the way the books say. It is. Children are told a great many stories that are polished in the retelling, and they benefit from adults who say so cheerfully and without fuss. "This is how it is usually told; one of the players said it stopped a little earlier" is a sentence that does no harm to the game and a great deal of good to the listener.

Something to do

Play the last three moves from the position after 20...Na6, with the child as White. If they find the queen sacrifice themselves, wonderful. If not, show it, and then ask them to explain why the knight had to take. The explanation is the lesson.