The FIDE rules clearly define the game of chess as a game where each player always has exactly one king. This follows fromhgm wrote:However, playing with two Kings does not violate any FIDE rule. AFAIK there are no rules for setting up positions.
- the definition of exactly one starting position where each side has exactly one king,
- the promotion rules that disallow promoting to a king,
- the rule that disallows capturing a king, and
- the various places where the term "the king", "his king" etc. are used, e.g. in the definition of checkmate, which unambiguously confirms that the rules are explicitly made for board positions with exactly one king of each color.
Whenever you set up an arbitrary chess board position, on a physical board or within a chess program, you are immediately faced with the question whether that position is reachable from the official starting position with a sequence of rule-conforming moves. There are obvious and non-obvious cases. Obvious cases where the answer is "unreachable" include those where the number of existing pieces of a certain type is already "out of range", e.g. more than 10 white knights or more than one black king. A couple of other simple indicators for obviously "unreachable" positions exist, e.g. pawns on ranks 1/8 or "out of range" piece counts when also considering the maximum possible number of promotions (= number of missing pawns) of a color. For the non-obvious cases you often need a retrograde analysis to decide about "reachability".
The question whether a chess program shall allow setting up a position that is definitely unreachable from the official starting position of the game must be answered individually for each engine. Opinions differ a lot, and I think everything between strict refusal of all obviously non-conforming positions (as in Gaviota for instance) and permissive acceptance of almost all positions you can think of (as in Micro-max for instance) can be acceptable. The permissive way can be seen as a kind of "FIDE rules extension" by allowing starting positions that are unreachable from the official one.
What I consider as incorrect, though, is to say that "playing with two Kings does not violate any FIDE rule". Furthermore, allowing to play with a number of kings different from 1 for any color immediately creates the need to redefine the semantics of the "checkmate" rule, which is a topic for which there is no accepted standard to my knowledge. I would even say that playing with two kings should be disallowed since that would be a game different from chess due to a different strategy (e.g. "give checkmate to ALL kings of the opponent").
Sven
