Kriegspiel (chess)
Kriegspiel is a chess variant invented by Henry Michael Temple in 1899 and based upon the original Kriegsspiel (German for "war game") developed by Georg von Reiswitz in 1812. In this game each player can see their own pieces, but not those of their opponent. For this reason, it is necessary to have a third person (or computer) act as an umpire, with full information about the progress of the game. Players attempt to move on their turns, and the umpire declares their attempts 'legal' or 'illegal'. If the move is illegal, the player tries again; if it  is legal, that move stands. Each player is given information about checks and captures. They may also ask the umpire if there are any legal captures with a pawn. Since the position of the opponent's pieces is unknown, Kriegspiel is a game of imperfect information. On the Internet Chess Club, Kriegspiel is called "Wild 16". Rules. There are several different for Kriegspiel. The rules offered on the "Chess Variant Pages" are as follows. The game is played with three boards, one for each player; the third is for the umpire (and spectators). Each opponent knows the exact position of  just their own pieces, and does not know where the opponent's pieces are (but can keep track of how many there are). Only the umpire knows the position of the game. The game proceeds in the following way: The umpire announces: Pawn promotions are not announced. The precise location of the checking piece is not announced (although it may be deduced). To avoid wasting time with many illegal pawn capture attempts, players may ask the umpire "Are there any pawn captures?" or just "Any?" If there are no legal pawn captures, the umpire answers "No". Otherwise, the umpire answers  Try! Asking "Any?" and receiving a positive answer obligates the asking player to then attempt a pawn capture: if this capture is unsuccessful, the asking player may then try any other move, pawn capture or not. "En passant" pawn tries are announced, but not the fact that they are "en passant" captures. Asking "Any?" when a player has no pawns left is treated as an illegal move and answered "Hell no" (or "Impossible", "Nonsense"). Illegal move attempts are not announced to the opponent. Kriegspiel problems. Kriegspiel is sometimes used in chess problems. In these, usual variations introduced by different black  moves are replaced by variations introduced by different announcements. An example of a Kriegspiel problem is shown at the right. White must checkmate Black in 8 moves, no matter where the black bishop initially is (it is somewhere on dark squares) and no matter what Black plays. (In a real Kriegspiel game, Black would not see White's moves, but for a problem in which White is to force a win, one must assume the worst-case scenario in which Black guesses correctly on each move.) For example, 1.Ra1 is a draw by stalemate if the black bishop was initially  on a1. 1.Nf2 Bxf2 2.Kxf2 (or Rxf2) is stalemate as well. So, White should not move either the knight or the bishop, because either might capture the black bishop by accident. For the same reason, the white rook should move only to light squares – but only half of the light squares are reachable without visiting a dark square along the way. Additionally, White should avoid placing his pieces on the a7–g1 diagonal prematurely because the invisible black bishop could be guarding that diagonal and capture the white pieces upon entering it, leading to a draw. The same applies to  the e1–h4 diagonal. The solution is the following: White tries to play 1.Rg2. White continues with 2.Rg8.