illegal moves in chess

Hey @chessmn13! 👋

Great question—this is something a lot of people wonder about early on!

So, yep, you already know the basics of how pieces move, but there are a few extra rules that define what counts as an illegal move in chess. Here are the big ones:

🚫 Putting or leaving your king in check. You’re never allowed to make a move that would leave your king in check. So, for example, if your bishop is pinned and moving, it would expose your king; that move is illegal.

♟️ Illegal pawn moves- Pawns can't move backwards
- They can only capture diagonally, never forward
- En passant is a special rule where a pawn can be captured "in passing"—but only under certain conditions
- Promoting a pawn to something other than a queen, rook, bishop, or knight is illegal

♜ Castling rulesCastling isn’t allowed if:
- The king or rook has already moved
- The king passes through or ends up in check
- There’s a piece between the king and the rook

🌀 Other stuff- You can’t make any move that breaks the standard movement rules (like trying to move a knight diagonally)
- In tournaments or official games, making an illegal move might cost you the game, depending on the ruleset!

Hope that helps clear things up! 👍

I'd like to add to the above reply:

1. You also cannot castle while in check.
2. En passant is where a pawn moves two squares, bypassing an enemy pawn's attack square.
   1. For example, the black f-pawn moving to f5, when there is a white pawn on e5 and/or g5.
      1. When this happens, on white's next turn — and ONLY their next turn — they can capture the black pawn as though it moved to f6.
3. Playing an illegal move usually results in the opponent receiving extra time on their clock, if it's called out. Repeated illegal moves will generally be considered an attempt to cheat and will result in an automatic disqualification (like resignation, only it's not of one's own volition. Disqualification can also happen if a player does not show up for a scheduled game, but that's unrelated).
   1. In rare circumstances, just one illegal move can be an automatic disqualification.
4. Even if a piece is pinned to the king, you still may not move your own king into its line of attack under any circumstances. Thus, an absolutely pinned piece can still participate in checkmating the king.