Re: The almighty dollar
Is the dollar truly freely convertible?
Yes. That's why it's the currency used for large chunks of the oil trade. If I sell a billion dollar's worth of oil to someone in China, and they pay me in Yuan, I can't freely spend that - because I'm not in Chain. More, I can't change that Yuan into Pounds, Euros or anything else, without getting approval from the Chinese government. And I'd imagine that's harder for a foreign company to do, than a Chinese one.
Also, it's common for lots of internationally traded commodities to have a price in a fixed currency. This means everyone knows what price they're talking about. So the International Metals Exchange is in London, and I believe quite a few metals are therefore priced in Sterling.
However there's no conspiracy here. If I want to buy a bunch of oil, I can pay in whatever currency is mutually agreeable. The price will still be set in dollars, according to the market price, but we can convert that into anything. Cuba paid Venezuela for their oil in a combination of not very good bodyguards for Maduro and doctors for the Venezuelan government clinics.
One of the other reasons for global currencies is trade imbalances. If you're buying natural gas from Qatar, they've got a pretty small economy. They aren't likely to be buying enough things in Europe to make up for the fact that they sell loads of LNG. So the Europeans don't have the Dihrams to buy that oil (not enough trade going in the other direction), and the Qataris might not want the Euros - so everyone agrees on dollars. If Qatar has a use for the Euros, such as buying sports teams or investing in the European capital markets, then they can do the deal in Euros.
Similarly, Russia is selling lots of oil to India, at some nice discounts. But they can't use the dollar, because of sanctions. So how to pay? India don't have an Rubles. India export foodstuffs, but that bit of the Russian economy is doing very well, so there's not need there. Some of the difference might be made up in sanction-busting, India imports stuff Russia can't get - and shifts it to Russia for the oil. Or they could trade in Yuan, but Indian companies can't get hold of that, because of Chinese capital controls. That's why people normally use the dollar, because everyone can find a use for those.