I believe Numquam was referring to the player who's flag fell being in check.
Correct. if we were to interpret "legal moves" like that, then white can checkmate with a single bishop in a rook vs bishop endgame as follows:
Note that it is not immediately checkmate after white's move. Black first gets the move, but then he is checkmated when he runs out of time. Also if white runs out of time in the starting position, then it is a draw.
@MARattigan:
Am I to understand from your reply to Numquam that you changed your mind about "the series of legal moves" to be one in which the legal moves need not alternate between the two colors? Or did I misunderstand your earlier comments? And if you did change your mind, did you also change your mind about the legal moves not needing to be playable?
Not at all. Terms that are defined should be taken exactly and only as in the definition. Terms that are not defined should be interpreted in English. So "legal move" appearing in the text means a move conforming with arts 3.1 to 3.9, because that is the definition, and means something different from "licit move" appearing in my posts which means a move conforming with arts. 1.1 to 5.2.3 if it relates to a game played under Basic Rules or arts 1.1 to 12.9.9 if it relates to a game played under Competition Rules. The word "series" is not defined so "series of legal moves" occurring in the text means simply a number of moves conforming with 3.1 to 3.9 placed one after the other. This ordering could be considered as conceptual or temporal. Since "legal moves" in the defined sense are not required to conform with arts. 4.3 to 4.3.3 or 4.5 there is no restriction on the colours of the pieces in such a series.
But the players can play only licit moves. Any series of moves made by the players (which, because arts. 1.1 to 5.2.3 and arts 1.1 to 12.9.9 both include 3.1 to 3.9, are necessarily "legal moves") must conform with art 4 and therefore alternate in colour.
My response to @Numquam was regarding article 6.9, the relevant part of which reads "... the opponent cannot checkmate the player’s king by any possible series of legal moves". As mentioned the players can play only moves with appropriately alternating colours. If it were instead "... no series of legal moves results in checkmate", then there would be no such constraint. Further, a series of legal moves from a position will result in a new position but because no player to move is defined in the new position (1.3 applies to moves played or assumed to be played in the game, but not to a conceptual series of legal moves) you would have to arbitrarily assume that to assert the resulting position is checkmate.
As for a legal move being playable, again not necessarily. I'm assuming "playable" here means licit. It's often used in a different sense in chess literature, but that's not relevant. The definitions of all "legal" moves assume a position that has some attributes of a position that would occur in a chess game (partial board layout at minimum). The definitions of some require those attributes to be attributes of positions in the game to which the rules are being applied or are well defined only if a player in the game is the mover of the piece.
But the definitions incorporate many possibilities that couldn't occur in a game because of articles other than 3.1-3.9 as well as moves that are prohibited by those other articles. In those cases the legal moves are not playable.
About checkmate I'd say that a checkmate cannot be established until the move is finished. For instance while playing a move one might move a piece via a square where the opponent would be checkmated but unless the move is completed at that point it can't be counted as a checkmate. If the flag falls before checkmate move is completed, it's therefore not checkmate and you lose (unless there is a minimal draw). If the checkmate move was completed before the flag fell it's a win for the checkmate giver. The appears to be not as much in the rules as in the observation of the event order which is a tricky thing.
I believe Numquam was referring to the player who's flag fell being in check.
If you agree with my interpretation of the checkmate definition in the preceding post then certainly a checkmate doesn't occur before a move is completed because it requires the checkmated player to 'have' the move.
There is a problem with the order of events occurring close together in a number of chess scenarios. According to Einstein this could depend on the velocity of the arbiter in direction connecting the players, resulting in variable possible results. In practical terms events would be taken as simultaneous if nobody can say which came before which. I would interpret 6.3.1 as being complied with if either the relevant number of moves or a terminating move occurred simultaneously with the flag fall, so 6.9 would not come into play.
It would be a lot simpler if FIDE would include a results table for "simultaneous" events.