The second rule governs castling:-
- The Origin of Castling in Chess
- More on Castling in Chess
- Davidson on the King's Leap
- Murray on the King's Leap, Spanish Style
- Murray on the King's Leap, Italian Style
I didn't pick these two rules at random. Together they constitute the sole difference between traditional chess (aka SP518: RNBQKBNR) and chess960. A frequent objection to chess960 is the emotional argument, 'It's not really chess!', which is only true if we consider chess to be exactly the game defined by FIDE's Laws of Chess (appropriately called 'FIDE Chess' by certain people).
The laws are found in FIDE's Handbook under section 'E. Miscellaneous': E.I.01A. Laws of Chess, where, contrary to the belief of the 'It's not really chess!' crowd, we also find in E.I.01B. Appendices the rules of chess960, which were added earlier this year. [Am I the only person who thinks it odd that FIDE classifies the laws of chess under 'miscellaneous'?]
In my lengthy quotes from Murray on the origins of the two rules, I included his summaries on other rules that have evolved through the centuries -- the moves of the pieces, the Pawn's initial move, Pawn promotion, en passant, stalemate, the bare King. These show that the similarities between traditional chess and chess960 are more significant than the differences.