Questions tagged [illegal-move]

A move that doesn't respect the rules of chess.

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32
votes
3answers
13k views

Can a piece pinned to my king put the opponent's king in check?

I was playing a game with a friend online. A situation came up where he put me in check with his bishop, and I moved my rook to block it. His king was on the g file and my rook was on g2. The website ...
4
votes
3answers
15k views

Moving king into check [duplicate]

Can a player move their king into check if the opponents piece that is able to take king, is putting opponents king in check?
20
votes
1answer
8k views

Why is it illegal to move into check?

In chess, it is against the rules to move into check, or to not move out of check. But why? Obviously, if you went against this rule, you'd immediately lose. But it seems strange to make it illegal to ...
28
votes
2answers
23k views

I made an illegal move. What happens?

Especially in blitz or bullet games, it is possible that a player makes an illegal move, for example castles through check. If the opponent does notice the illegal move, then as far as I know the ...
33
votes
2answers
16k views

Is taking the opponent's king an illegal move?

In competition blitz games, making an illegal move and then pressing the clock forfeits the game if the opponent claims the illegal move as long as he does not make a new move, as a new move would ...
8
votes
1answer
620 views

Illegal Move Missed-Too Late?

This question is about standard games under USCF rules (if FIDE is different, I'd like to know). Say that one side makes an illegal move-specifically, castling through check. Neither player notices. ...
18
votes
6answers
4k views

How to know when a FEN position is legal?

I am doing a personal project, where at one point I need to validate the FEN position, I started with some basics checks, such as check if there are kings and making sure there aren't any extra rows ...
19
votes
6answers
3k views

Are there any illegal positions that are difficult to spot?

I have a question and I'm sorry if it sounds very uninformed, I'm not a professional chess player so my interest is mere curiosity. I was wondering if there are examples of illegal positions that ...
21
votes
4answers
28k views

Why is castling through and out of check prohibited?

Why is castling disallowed when the king is in check or when the square the rook will be placed on is under attack? What is the rationale behind this ruling?
5
votes
2answers
944 views

Detect pinned pieces code

This post is a follow up of this one: C++ vs Java Engine move generation performance My goal here is to improve the performance of my move generator, which finishes perft (start_pos, 6) in 8.5 secs. ...
2
votes
1answer
236 views

What happens if a player makes an illegal move and nobody notices?

What happens if a player makes an illegal move (i.e. a realistic case could be, he castles after he already moved the King), but none of the player notices, and the game goes on and on for a lot of ...
1
vote
2answers
179 views

In a tournament game, what should a player do if he find that his opponent's position is impossible, which was caused by an earlier illegal move? [duplicate]

For example, if a player realises that his opponent has two bishops of the same colour after a certain legal move (and assuming that it is clearly not caused by any pawn promotion), which implies that ...