user12762: 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?

user14309: 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.

user825: 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.

user7297: 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.

user12401: 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.

12762: <p>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?</p>
 14309: <p>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.</p>
<p>I think at least as compared to things like ?! or !? it makes a lot of sense.</p>
<p>Oddly, it's the only carry-over from the Descriptive (P-K4) to Algebraic (e4) notation.</p>
 825: <p>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.</p>

<p>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.</p>
 7297: <p>A notation like Kg1 would give the impression that only the king is moved. At the very least, it is not <em>obvious</em> that castling moves the rook as well.</p>
 12401: <p>The castling notation was invented by Johann Allgaier and used for the first time in his 1811 2nd edition of his <em>Neue theoretisch-praktische Anweisung zum Schachspiel</em>.</p>

<p>He didn't explain why he came up with it.</p>

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