In a Paris coffee house in the middle of the eighteenth century, the strongest player in the room left his queen where anyone could take it. His opponent took it. Two moves later the game was over.

That is Légal's Mate, the oldest chess trap still worth teaching. It is seven moves long. It has been played in parks, school clubs and online blitz games ever since, and it keeps working, because it is built not on a weakness in the board but on a weakness in the way people use rules.

The game in three movements

The claim. On his third move, Black pins White's knight to the queen. The knight now stands between Black's bishop and White's queen. Every beginner learns the rule: don't move a pinned piece. Black is counting on it.

The refusal to obey. On move five, White moves the pinned knight anyway, capturing a pawn, and leaves the queen hanging. If Black declines the queen and takes the knight, White takes Black's bishop and is simply a pawn up. The pin was never really holding anything. If Black takes the queen, it is worse.

The net. Black takes the queen. White's bishop takes the pawn on f7 with check. The black king has exactly one square to go to. A knight lands on d5, and it is mate, delivered by two knights and a bishop, while the bishop that took the queen stands uselessly in the corner of the board.

The man in the café

It helps to picture the room. A coffee house with marble tables, newspapers on poles, pipe smoke, and in the corners men bent over boards for hours, some for stakes and most for the pleasure of it. It was one of the few places where a nobleman and a clerk might sit down as equals, if only for the length of a game.

The Café de la Régence was, for a long stretch of the eighteenth and nineteenth centuries, the centre of the chess world. Anyone could walk in, order a coffee and sit down to play. Kermur de Légal was its strongest regular for years. It is said he often gave odds, removing one of his own pieces before a game to make it fair, and that in this game he played without a rook. It never moves in the published score, so the story cannot be checked from the moves.

His opponent, Saint Brie, is known to history for this one loss. That is a hard way to be remembered, and a little unfair: the move that lost was the most natural move on the board.

What is known, disputed and legendary

• Known. The trap itself, which has been played countless times since, and the moves as they are usually published.
• Disputed. The date, traditionally 1750, possibly the 1780s. And the move order: research suggests the original game ran a little differently, in a way that gave Black a good reply, and that the version in the books was tidied so the combination works against anything.
• Legend. That Légal disguised his trap by touching his knight, then pulling his hand back as if he had only just noticed it was pinned. It is a lovely detail. It is told rather than recorded.

Why it is worth an evening

Most chess lessons are about what to do. This one is about when to stop trusting what you were told to do. "Don't move a pinned piece" is a good rule. It is true most of the time. Black lost because he built his whole plan on it without asking whether it was true in this position.

That is a habit worth practising away from the board as well. Any rule of thumb is a short way of remembering something that is usually so. The useful question to keep asking, cheaply and often, is "what happens if it isn't, this time?" Most of the time the answer is "nothing". Légal's Mate is what happens on the other occasions.

How a 250-year-old game gets checked

It is fair to ask how anyone knows any of this. Nobody filmed the Café de la Régence. The answer is the same as for any history: you look for what was written down closest to the time, and you trust it more than what was written later.

The moves of this game reached print after it was played, and they have been reprinted ever since, often without comment. Researchers who have gone back to the older sources found reasons to doubt the traditional date, and a likely original move order that made the combination unsound. The cleaner version survived because it is the better lesson. That happens a great deal. Stories that teach well get copied, and each copy is a chance for someone to tidy them a little more.

None of that makes the game less worth knowing. It makes it a good example of how to hold a story you love. You can enjoy the trap and teach it and still say, plainly, "this is how it is usually told, and the original was probably a little different." Children are good at this distinction when grown-ups model it. They hear "once upon a time" every night and know exactly what it means.

Why traps are good teachers

There is an old worry that teaching children traps teaches them to play for cheap tricks. In practice it tends to do the opposite. A child who has been caught by Légal's Mate once becomes suspicious of free material for a long time afterwards, and suspicion of free material is one of the most useful habits a chess player can have. A child who has sprung it once learns that the most valuable piece on the board can be worth less than a tempo and a square.

Both lessons are about value being contextual. A queen is worth nine points in the textbook. In this position, for Black, it is worth less than nothing, because taking it costs the game. That idea transfers to plenty of things that are not chess, and it is easier to learn from a seven-move game than from a lecture.

The question for the table

If you play the trap through with a child, stop before White's fifth move and ask: "The knight is pinned. Is it really stuck?" Let them try moving it and see what each reply gives. Then ask the harder one, the one the gleaner in the children's version learns to ask: "If something looks free, why is it there?"

Your child may have just read the same game as Légal's Mate at the Slow Lake Academy, where a hen named Merle takes the queen because nobody is guarding it, and a weaverbird named Linnet closes the net in two moves. In that version the girl prompting the play learns to ask her question before she names what she sees.

Something to do

Play the seven moves with your child taking White. Then swap. When you play White, leave the queen hanging, and see whether your child takes it. If they don't, ask what made them suspicious. If they do, play it out to mate, and then set it up again and let them try it on someone else. A trap you have fallen into once is a trap you remember. And a trap you have set once, and watched someone walk into, teaches you something about the person across the table as well as about the board: what they were looking at, and what they were not. That is worth a conversation of its own, afterwards, over whatever is left in the biscuit tin.