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USD is also much less volatile than BTC, so easier to gage what the crypto is doing rather than what BTC is doing. Obviously trading between X and BTC you need to be aware of both BTC & USD, but otherwise, if you're investing from USD in the first place then that's going to be the most relevant comparison for you.
Example: I invest from GBP, fiat trade with EUR, crypto trade with BTC, but use USD as the reference... because the mid-term difference of EUR/GBP/USD is so minimal compared to cryptos.

Short answer is he always trades back to USD for safe keeping of profits. Also, rarely do the charts vary much unless there is almost no volume in one or the other.

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