Apparently it’s allowed under USCF (US Chess Federation) rules, according to my sibling comment, just not under FIDE (international) rules.
Anyway, I’ve never stopped and thought about why it’s not allowed — it just seems like it obviously shouldn’t be, in serious competition. If at an NBA game they ran out of basketballs, they’d stop the game until they got one, not use a soccer ball instead.
It’s hard to imagine that at an actual FIDE-rated tournament with arbiters, etc., they would be unable to find a queen piece to use.
> it just seems like it obviously shouldn’t be, in serious competition
Why does it seem obvious? Out of some sense of accessibility to third party observers?
> If at an NBA game they ran out of basketballs, they’d stop the game until they got one, not use a soccer ball instead.
This is an unreasonable straw man because basketballs and soccer balls behave quite differently. A marble would be less suitable than an overturned rook because it may roll away, but both are similarly graspable with similar dexterity.
> Why does it seem obvious? Out of some sense of accessibility to third party observers?
I can think of lots of reasons.
1. It looks cheesy and unprofessional to use random objects instead of the pieces the game is supposed to be played with; you might not think this is a good reason but keep in mind we are talking about a game that until recently everyone played wearing a suit and tie.
2. It is distracting and impedes comprehension and calculation if the design of the actual pieces is burned into your pattern recognition — not only for observers, but for the players themselves. A lot of official chess rules, e.g. the touch-move rule, are just about not annoying your opponent.
3. It opens up ambiguity about what was actually intended. What if later the player tries to claim they really did mean a rook? What if a player accidentally turns one of their actual rooks upside down during the course of a game — is it still a rook, or are they trying to cheat by turning it into a queen? Etc.
4. It does not work with high-end electronic chessboards that automatically record moves (DGT).
5. Last but not least: there is absolutely no reason to allow this, because it’s impossible to imagine that at a serious tournament the arbiter wouldn’t be able to find an extra queen. And stopping the clock and asking for an arbiter, while still a bit distracting to others, is surely less distracting than starting a discussion with the opponent about whether it’s okay to use an overturned rook or any other random object as the queen.
i can definitely see someone stalemating with the rook-queen then claiming its actually a rook so its not a stalemate.
In most board games, proxy pieces are generally forbidden in official play. For say, a card game it's because the game store owns the cards and would really prefer it if you didn't dillute the value of the cards they're also selling to people, not to mention they also have the actual cards in stock anyway. On a similar note, chess tournaments and clubs will almost always have enough pieces to not need proxies since there's only 4 unique pieces that you'd potentially need to add for (all the officials minus the king, in practice it's usually only the Queen and the Knight though), so any extra/reserve chess set can provide the bonus piece.
In casual play outside of formal tournaments and chess clubs, proxy pieces exist because nobody is buying extra chess kits solely to cover for the event in which someone promotes a queen while another queen is on the board. (Also in very casual play, most players lose their Queen due to a mistake early on and if they promote a piece to Queen later, they just use the original Queen piece again.) Proxy pieces exist to cover people playing at home, not people playing professionally or at a hobby club. The same goes for card games; nobody cares if you're proxying a card during casual play - maybe they'd ask if you own the card, but that's about it.