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Most exchanges that have coins other than Bitcoin use Altcoin/Bitcoin pairs. So you're not trading in relation to the US Dollar, you're trading in relation to Bitcoin. For a trader, that means that to them 1 ETH is currently worth 0.02678663 BTC. The USD price of Bitcoin doesn't come into play here. So, if Bitcoin goes down, the relation to ETH means that it follows, since 1 ETH is still worth 0.02678663 BTC. Edit: Why would they do this? A number of reasons, the biggest two being habit and volume. Most coins are traded on networks that don't have fiat currencies, so they had to choose a cryptocurrency base. Also, many traders in the realm started out with Bitcoin, and love Bitcoin, and want more Bitcoin. So they don't care if a trade gets them USD, they just want to grow their BTC holdings. Finally, with trading, volume is very important - you want as much being offered for sale as possible. If you split up every order book (btc/eth, xrp/eth, ltc/eth, usd/eth, xrp/ltc, xrp/usd, ltc/btc, etc etc) then the volume of each 'book' is divided amongst the options, making it harder to move larger amounts from one to the other. |