Efficiency of opening tempos.
Why it matters
Every move in the opening is a chance to bring out a new piece. Moving the same one twice spends that chance, and after a few of them the opponent has an army out and you have one piece on a nice square.
The exception is when moving it twice gains something concrete — a capture, or a square you could not otherwise have. What the rule really says is: know what you are paying for.
See it on a board
Black's knight went to e4 and straight back to f6, which loses a move and walks into the discovered check. Two knight moves, one queen.
Checked: this line wins at least 5 pawns of material.
How to play it
- Before moving a developed piece again, ask which piece is still at home and whether it could move instead.
- Accept the second move only for something concrete: material, a tempo on the enemy queen, or a genuinely better square.
- Punish the opponent's repeated moves by opening the position while they are still shuffling.
At each level
The same idea, said the way it is taught at each age.
Pawn · ages 3–6
In the opening, move each piece once. Bring out a new piece instead of moving the same one again.
Knight · ages 4–6
The rule has exceptions and they are all concrete. If you cannot say what the second move wins, it is a wasted move.
What goes wrong
- Retreating a developed piece to its starting square because it felt awkward.
- Chasing the opponent's pieces around with the same knight while the rest of your army waits.
Questions people ask
- What about openings where a piece moves twice on purpose?
- They exist — the Ruy Lopez bishop, for instance, moves several times. In each case the moves come with tempo or reach a square that cannot be reached otherwise, which is exactly the exception the rule allows.