The rules for Fischer Random and Chess960 are essentially identical to Classic Chess, except for castling.
Fundamentals of castling in Fischer Random Chess and Chess960
King and rook initial placements
All Fischer Random starting positions must have the king between the two rooks to allow castling in both directions.
Regular castling rules
Just like in regular chess, players cannot castle out of check, and the king must not cross or land on an attacked square. Both king and rook must not have moved prior to castling.
“King-side” & “queen-side”
In the examples below we refer to the king-side and queen-side, using the terms and definitions used in regular chess. We keep these terms in place, regardless of where the king and queen start.
Special castling rules for Fischer Random Chess and Chess960
Position of castled pieces
When castling, both the king and rook end on the same squares as in regular chess, regardless of the starting position. In the position below, White has castled king-side and Black has castled queen-side..
Initial position of kings and rooks.
White has castled king-side and black has castled queen-side.
Castling path must not be blocked
All squares between the king and rook must be empty before castling.
In the following starting position, White must clear f3 before castling king side, while Black must clear the three pieces between the king and rook prior to castling queen-side. Notice that the bishop on h1 does not interfere with White castling here.
Starting position.
Both sides are ready to castle on opposite sides.
Landing squares must not be blocked
In addition to the squares between the king and rook being cleared, the ending squares must also be vacated. Here, White is ready to castle king-side, and Black is allowed to castle queen-side.
Starting position.
White is ready to castle 0-0. Black is ready to castle 0-0-0..
Special cases to consider
The examples below adhere to the rules above, but may cause confusion while playing.
Swapping the King & Rook
Sometimes castling can be done on move one, simply by swapping them. In both diagrams below, White has castled, while Black has not moved yet.
Case 1: Position after 1. 0-0
Case 2: Position after 1. 0-0-0
Blocked by the rook
In this starting position both rooks are on the squares that the kings will land on after castling. In this case both pieces are just placed on the final squares as usual.
Initial position of kings and rooks.
White has castled king-side and Black has castled queen-side.
Castling while playing Fischer Random or Chess960
Castling in Fischer Random games is performed slightly differently depending on the format.
Chess.com & lichess.org
Just as in regular games online, simply drag your King to the square your rook is on.
Over the board (OTB)
According to the FIDE Laws of Chess:
When castling on a physical board with a human player, it is recommended that the king be moved outside the playing surface next to his final position, the rook then be moved from its starting position to its final position, and then the king be placed on his final square.
To avoid any misunderstanding, it may be useful to state “I am about to castle” before castling.