Working out what your opponent wants to do and stopping it before it happens, rather than answering it afterwards.
Why it matters
Prophylaxis is spending a move on what the opponent wants to do rather than on what you want to do. It feels like a wasted move and it is usually the strongest one on the board.
The reason is arithmetic. Stopping an idea costs one move; letting it happen and then dealing with it costs two or three, and often costs a concession as well.
See it on a board
Black would like ...Bb4, pinning the knight and forcing awkward decisions. a3 costs one move and takes the whole idea off the board before it exists.
Checked: the position really has the feature described.
How to play it
- Before choosing a move, ask what you would play if it were the opponent's turn.
- If that move is genuinely annoying, look for a way to make it impossible rather than a way to answer it.
- Prefer prophylactic moves that also improve something of yours, so the move is not purely defensive.
At each level
The same idea, said the way it is taught at each age.
Rook · ages 6–9
Ask what your opponent wants to play next. If the answer is annoying, stopping it is usually worth a move.
King · ages 10 and up
The strongest prophylaxis is against plans, not moves. Taking a square the opponent's pieces need is worth more than parrying a single threat.
What goes wrong
- Defending against threats that do not exist, and drifting while the opponent improves.
- Only looking at your own plan, and finding out what the opponent wanted after it has happened.
Questions people ask
- Is prophylaxis just being passive?
- No. Passive play answers threats after they arrive. Prophylaxis removes them in advance, which usually leaves you a move ahead rather than a move behind.