I might trade weed for propane, and the exchange rate might be geographically different, but generally speaking the rate is always track with the USD fiat denomination of each asset. USD denominated because that's the Units most things are Accounted for in. Any time the unit pair deviates from the underlying UOA values, it's an opportunity for a trader to step in and make free fiat. If weed is cheap versus propane, I can buy weed, flip it to propane, and sell it for free fiat. This action *forces* the trading pair to tend towards the underlying fiat UOA values. Basic economics.
That's actually quite incorrect. Basic economics. Read up on "Purchasing Power Parity", and then throw AML/CTF and sanctions into the mix.
bye
Wikipedia fixes this!
show your work or GTFO
Its framed on the wall of my office