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Keeping a smaller currency constantly pegged to a larger national one, to a regional one or even to a global reserve currency, is a difficult task which many central banks around the world have failed to do in the past in times of crisis. When enough speculators attack the currency, or capital outflows are just too great to handle, keeping the peg becomes unsustainable and it cracks. Despite this the people behind one controversial altcoin, Tether, are trying to achieve just that.
https://news.bitcoin.com/bitfinex-faces-further-scrutiny-over-tether-liquidity-concerns/