When is money used as a place holder rather than having monetary value?
In the game of pool, there are social conventions that exist regarding the desire to use the pool table next. One places a coin (e.g. a quarter) on the table above where the slot exists to insert money. On Mondays at this establishment, the pool table was free to use, yet the convention is still being followed.
Relevance: Is this convention unique to pool? Is it universal in other cultures outside of North America?