Smothered mate and Philidor's legacy

In this pattern the losing king is not trapped by the winner's pieces. It is trapped by its own.

A smothered mate is checkmate delivered by a knight when the losing king is surrounded by its own pieces. A knight is the only piece that jumps over other pieces. A wall of friendly pawns and rooks keeps a queen or a rook out, but it does not keep a knight out. So the wall the king is hiding behind is the reason it cannot escape.

The usual picture is simple. A king sits on h8 or h1 with two pawns and a rook beside it. One enemy knight lands on f7 or f2.

Philidor's legacy is the name for the forced sequence that produces a smothered mate. It runs like this, with White to play:

1. Queen gives check on the long diagonal. The king is forced into the corner.
2. Knight checks on f7. The king comes back out to g8.
3. Knight goes to h6 with double check. Two pieces give check at once, so the king must move. It has only the corner.
4. Queen goes to g8 and is given away. Giving a piece away on purpose is called a sacrifice. The rook must capture it, because the king cannot.
5. Knight returns to f7. Checkmate.

Every black reply in that list is the only legal move. That is what makes the pattern so strong.

A double check is a check from two pieces at the same time. It happens when one piece moves, gives check itself, and opens a line for another piece behind it.

Against a normal check you have three answers: take the checking piece, block the line, or move the king. Against a double check only one answer is left. You must move the king. You cannot capture two pieces in one move, and blocking one line leaves the other.

In Philidor's legacy the knight jumps to h6. The knight checks the king, and the queen's diagonal opens behind it. Black's rook cannot help, so the king must step into the corner.

Look for all four of these signs before you start calculating:

1. The enemy king is on a corner square, usually after castling.
2. The pawns in front of the king have not moved, so there is no escape square.
3. A rook sits next to the king on the back rank.
4. You have a knight that can reach f7 or f2 with check, and a queen that can reach the long diagonal.

If all four are true, check the sequence. If the king has a free square, for example because a pawn moved to h6 or g6, the pattern usually fails.

The defence is simple, and it is worth doing early:

• Make an escape square. Push the h-pawn or the g-pawn one square. This is often called making luft, which is a German word for air.
• Move the rook off the back rank, or move the king off the corner, if you see an enemy knight coming.
• Watch the long diagonal. If the enemy queen can reach it with check, count the sequence yourself before you allow it.

One warning. Pushing a pawn also creates new weaknesses. Push a pawn when you see a real threat, not on every move.