In most opening gambits, a pawn is worth two tempi(Two moves). So I guess that a pawn would be fine. To be more precise, the f pawn. I think that is the most logical way.
But yet there remains a problem. I will completecan mate you like this:
[Title "White to move"]
[fen "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]
1.e4 e5 2.Bc4 Bc5 I use the move now 3.Qf3 h6 4.Qxf7#
I used h6 as a waiting move to show that a double move is made.
In a similar way, a player can trap or take a big piece, such as a queen.
So the rules so far:
- White(Or black) lacks his f pawn.
- The person with the right to double move cannot mate. If a mate is inevitable, he cannot use the double move.
- The person with the right to double move cannot take a piece. If taking a piece is inevitable, he cannot use the double move.
About check now. I do personally think that if you are checked, the double move doesn't count, so:
The person with the right to double move cannot check. If a check is inevitable, he cannot use the double move.
[Title "White to move"] [fen "rnbq1rk1/pppp1p1p/5BpQ/4p3/4P3/8/PPPP1PPP/RN2KBNR w KQ - 0 1"]
But there are also positions like this. So this idea comes up:
- Right after the double move, the non-double move player can exchange one of his pieces for a piece of equal value. You can exchange a bishop for a knight. You cannot exchange say, two rooks for a queen and pawn.
So I think that this is the best way to answer a bit lateryour question.
Hope it helps.