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Open diagonals to the king are a potential weakness that can be exploited in the opening. I would say that following the typical lines for at least the first few moves of any established good opening would mean that this isn’t necessarily an issue. Simply, they wouldn’t otherwise be considered good! However, once you’re out of the first part of the opening, or, if the game deviates into unusual territory, keeping an eye out for these diagonals to the king are really important. In terms of our visual perception, we’re simply not as good at seeing diagonals compared to up and down, left and right.

I’m going to demonstrate the principle with a very quick game against one of the beginner chess.com robots that played a bad opening, and then, in a game against a human opponent.

As we see in the Queen’s Gambit, the early move c4 means that this pawn can no longer be part of the defense of the dark square diagonal on the queenside. Once the queenside knight and bishop had also moved for an early attack, this left this diagonal weak. At the beginner-intermediate level, there is also a psychological impact. Once I was above 1100 ELO or so on chess.com, most opponents intuitively understood that early queen moves are often bad, outside of established openings like the Scandinavian Defense. What this means is that a strike like this is often unexpected and unnerving. In this game, my opponent responded inaccurately to the first diagonal attack, and then missed the second devastating attack which pinned their queen to the king.

I hope you found this game interesting, and thanks for watching!

Game 1 on chess.com (Jimmy-bot): https://www.chess.com/game/computer/10284975

[Event "vs Computer"]
[Site "Chess.com"]
[Date "2022.04.25"]
[Round "?"]
[White "vitualis"]
[Black "Jimmy-Bot"]
[Result "1-0"]
[ECO "B00"]
[WhiteElo "1282"]
[BlackElo "600"]
[TimeControl "0"]
[EndDate "2022.04.25"]
[Termination "vitualis won by checkmate"]

1. e4 h5 2. d4 f6 3. Bd3 d6 4. e5 Nd7 5. Bg6# 1-0

Game 2 on chess.com (human opponent): https://www.chess.com/game/live/44506779339

[Event "Live Chess"]
[Site "Chess.com"]
[Date "2022.04.23"]
[Round "?"]
[White "qdab22"]
[Black "vitualis"]
[Result "0-1"]
[ECO "D10"]
[WhiteElo "1226"]
[BlackElo "1277"]
[TimeControl "1800"]
[EndTime "19:46:24 PDT"]
[Termination "vitualis won by resignation"]

1. d4 d5 2. c4 c6 3. Nc3 Nf6 4. Nf3 Bf5 5. cxd5 cxd5 6. Bg5 Ne4 7. Nxe4 dxe4 8. Nh4 Nc6 9. Nxf5 Qa5+ 10. Qd2 Qxf5 11. Be3 e5 12. dxe5 Bb4 13. f3 Bxd2+ 14. Bxd2 exf3 0-1