How does a currency pair work?

If the price in the ETH/USD pair is 3,000, this means:

1 ETH = 3,000 USD.

And if the price in the ETH/BTC pair is 0.05, this means:

1 ETH = 0.05 BTC.

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Uses of Ethereum pairs:

Trading: Buying and selling Ethereum against other currencies.

Conversion: Between cryptocurrencies or traditional currencies.

Making profits: By speculating on price changes between currencies.