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I'm aware that if one's flag falls then the game is declared a win for the opposing side if something like "they have enough material to mate" or "it is possible to mate". I'm curious about an edge case which actually differentiates between the above two scenarios, i.e. the opposing side has enough material to mate but it's impossible to mate from that position (for whatever reason).

I think an example would illustrate best the situation I had in mind. Suppose White has a King on a1 and a Queen on a2, while Black only has a King on h1. If White then plays Qh2+ (perhaps in an online bullet game) and Black's flag falls, does White win? Note that from this position it is impossible for White to mate, since Black's only legal move here is to play Kxh2, which would be an immediate draw.

There are other scenarios which would be impossible to mate, e.g. if White has a King on a1, pawns on a4, c4, e4, g4, and Black has a King on h8, pawns on a5, c5, e5, g5 -- clearly neither side can mate from this position. (This sort of situation is slightly less interesting to me as this is position that is less likely to arise than the one described above, but I'm still curious.)

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    The rule does not say anything about "enough material". If a flag goes down, his opponent will win the game if there is a sequence of legal moves that leads to its victory. Otherwise it's a draw.
    – David
    Oct 28 '20 at 10:04
  • Note that while this should be a draw according to the rules, this doesn't mean that it will be counted as such in an online chess game. Example: lichess.org/MEndLrYy
    – xehpuk
    Oct 28 '20 at 21:27
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From FIDE Laws of Chess, article 5.2b:

The game is drawn when a position has arisen in which neither player can checkmate the opponent’s king with any series of legal moves. The game is said to end in a ‘dead position’. This immediately ends the game, provided that the move producing the position was legal. (See Article 9.6).

In your first example, as soon as White plays Qh2+, the statement "a position has arisen in which neither player can checkmate the opponent’s king with any series of legal moves" becomes true. As the rule says, this immediately ends the game with a draw. This happened on White's move and it immediately ends the game, so there is no time for Black's flag to fall.

The same happens in your second example. As soon as a dead position arises, it is a draw.

The position does not need to be dead for a fallen flag to result in a draw. Article 6.9 says:

Except where one of the Articles: 5.1.a, 5.1.b, 5.2.a, 5.2.b, 5.2.c applies, if a player does not complete the prescribed number of moves in the allotted time, the game is lost by the player. However, the game is drawn, if the position is such that the opponent cannot checkmate the player’s king by any possible series of legal moves.

So if Black's flag falls and there is no legal sequence of moves that results in Black's king being checkmated, it is a draw. For example, if Black has an extra queen in your first example, and Black's flag falls after this move

[FEN "8/8/8/5q2/8/8/Q7/K6k w - - 0 1"]

1. Qh2+

then it is still a draw, because there is no legal sequence of moves can result in Black's king being checkmated.

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  • It's even true if instead White has an extra B or N (as long as it doesn't support Qh2+ . . .), or White has an extra B/N and Black has an extra Queen. Any other Black piece wouldn't do, because a mate could be constructed; even B vs. P or N vs. P is enough mating material because (if the P is not already on a- or h-file) it could underpromote to a Knight and smother the bK in the corner! Exception: K+B vs. K+B is a dead draw if the Bishops move on same-colored squares (this used to be noted explicitly in the rules). Oct 28 '20 at 22:25

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