Progressive chess

In progressive chess, the number of moves available grows on every turn. White makes one move, Black makes two, White makes three, Black makes four, and the pattern continues. The complete group of moves made by one player is called a turn or series.

Every intermediate move must be legal. A player cannot disregard king safety during a series and repair the problem at the end. If the turn begins in check, meaning that the king is under attack, the first move must remove that check.

When a series ends

Under the Scottish rules, one of the best-known forms, a series ends as soon as a move gives check to the opposing king. Any unused moves are lost. The king is not captured: the objective remains checkmate, a position in which the king is attacked and has no legal reply.

If a player has no legal move at the start of a turn, or runs out of legal moves before completing the required series, that player is stalemated. Stalemate means having no legal move while the king is not in check, and it produces a draw under this rule set.

How calculation changes

• A threat must be judged against the length of the opponent's next series. A defense that survives two moves may fail against five.
• Giving check can close a series early. It is often useful to reserve check for the last available move.
• The same piece may move several times during a series as long as every step is legal.
• Material can change rapidly because one player may make several captures before the opponent intervenes.

There is no single universal rule set. Scottish, Italian, and English forms are known, with differences in the treatment of check and other details. The increasing sequence is the shared idea, but the exact version should be stated before play.

Progressive chess resembles Marseillais chess because one turn may contain several moves by the same player. The central difference is that Marseillais chess keeps a fixed allowance of two, while progressive chess continually lengthens the series.