Back rank and mating nets

The engine's tag walks into mate covers a specific kind of loss: the game was not decided by material, it was decided by a king with no squares. On the pattern page you can see the surprising part: this mistake does not fade with rating the way hanging pieces do. It happens to players who are winning. They stop checking their own king because the game is “over”.

The back rank

The pawns that keep your king safe also keep it in.

That pawn move has a name, luft (air). The rule: once the queens or the heavy pieces are on open files and your king is behind an unbroken row of pawns, make the hole before you need it. The right moment is usually a quiet one, when you have nothing urgent to do. The wrong moment is when the opponent's rook is already on the open file, because then it is a tempo you may not have.

Two refinements:

• h3/h6 is usually right, g3/g6 is sometimes right. The h-pawn move keeps the diagonal towards the king closed. Moving the g-pawn opens it. Pick based on which enemy bishop is alive.
• A defender on the back rank is not luft. A rook on f1 “covers” e1 until it is deflected, overloaded, or captured with check. Count how many things that rook is doing. If the answer is two, you do not have a back rank, you have a problem.

Mating nets

A net is the same idea without the rank: the king has squares, but every one of them is covered, and one more piece arrives.

Nets follow a small number of shapes, and it is worth knowing them by sight rather than calculating them every time:

• Queen next to the king, defended. On the h-file or the seventh rank, with a piece backing it up. If the queen can land on h7 or g7 with support, that is the threat to look for.
• Rook on the seventh with the king on the eighth. The king is confined to the back rank; a second piece finishes.
• The smothered king. A king boxed in by its own pieces. Only a knight is needed, because only a knight attacks without needing a line. This one belongs in the next article, because it is the win everybody misses.
• Two lines crossing. A bishop takes one diagonal of escape squares, a rook or queen takes the file. The king stands where they cross.

The thing about nets is that the last move is rarely the surprising one. The surprising part is that the square the king wanted three moves ago was already gone.

The tell

Your king has two or fewer legal squares, and an enemy heavy piece can reach an open line towards it within one move. That is all. It sounds obvious, and it is the exact position players are in when they get mated while winning.

The habit

Once per move, especially when you are ahead: "Does my king have an escape square, and can any enemy piece give check next move?" If the answer to the first is no, make one now. If you cannot spare the tempo, at least know that you are playing with fire and calculate the check.

And the one that costs nothing: when the opponent's rook or queen lands on an open file, look at your back rank that move, not the next one.