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I'm curious about why the symbols O-O and O-O-O are used for castling in algebraic notation. Why not use the consistent and logical Kg1/Kc1 for White and Kg8/Kc8 for Black? Why adopt the less than obvious O-O and O-O-O from descriptive notation?

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    0-0 and 0-0-0 are more "obvious" if you realize it's the number of squares the rook moves. (Comment, because it's really not a full answer.)
    – Ghotir
    Mar 10, 2017 at 19:44
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    BTW, it should be 0-0 (with the number zero, not the letter O). Only in PGN it has to be the letter.
    – user1583209
    Mar 10, 2017 at 21:30
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    According to wikipedia this notation was introduced in 1811 by Johann Allgaier as 0-0r, 0-0l. The current notation with 0-0-0 was added in 1837 by Aaron Alexandre.
    – user1583209
    Mar 10, 2017 at 21:33
  • Better would be CQ and CK, instead of Kc1 and Kg1.
    – Fred Knight
    Sep 15, 2017 at 9:45

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The castling notation was invented by Johann Allgaier and used for the first time in his 1811 2nd edition of his Neue theoretisch-praktische Anweisung zum Schachspiel.

He didn't explain why he came up with it.

Allgaier (and algebraic notation in general) used digit-0. The use of letter-O is an anglo-saxon oddity.

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  • I always thought of the kingside notation as being a couple of circles with a line between, representing the bottoms of the king and rook, and the queenside notation was a "bigger" version of that. I've never thought there was any relation to the number zero.
    – supercat
    Aug 23, 2018 at 23:19
  • @supercat there is no relation to the number zero it is the letter 'oh' and pronounced oh-oh or oh-oh-oh never zero-zero or zerozerozero like the movie title.
    – edwina oliver
    Feb 19, 2020 at 17:01
  • @edwinaoliver I don't think so. For instance, Spanish uses 0-0-0 ("cero-cero-cero")
    – David
    Dec 10, 2020 at 15:26
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A notation like Kg1 would give the impression that only the king is moved. At the very least, it is not obvious that castling moves the rook as well.

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The point is to make it obvious it's a castling move. It's important because that's the only time in a game that you can move two pieces.

Note that computers represent the moves as Kc1 and Kg1, so both ways work. O-O and O-O-O are easier for humans to read.

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In my opinion, 0-0 and 0-0-0 are used to differentiate castling from ordinary king moves. The castling maneuver then stands out in the game notation, as opposed to say Kc8 which appears - on the surface anyway - to be an ordinary, one square, king move ... until you look closer to see if the king is actually moving more than one square. It also helps to clearly indicate king-side (short) or queen-side (long) castling at a glance.

I think at least as compared to things like ?! or !? it makes a lot of sense.

Oddly, it's the only carry-over from the Descriptive (P-K4) to Algebraic (e4) notation.

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Kg1/Kg8 or Kc1/Kc8 are Singular moves where the King only moves during the Game . Since before the Algebraic Notation Descriptive Notation was used which was quite cumbersome & lengthy Algebraic was a short hand . Why 0-0 & 0-0-0 was exactly used is quite inexplicable but this is the only move in International Chess where two pieces are moving simultaneously . In K-side Castling K is moving two squares and in Q-side Castling it is moving three Squares .

So it might have seen an innovative way of recording the Castling move since King is the most important piece in a Chess Game 0-0/0-0-0 was a royal move indicating King is going inside the castle and the Battle Begins .

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    If the K moved three squares in Q-side castling it would end up on b1.
    – bof
    Sep 15, 2017 at 12:36

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