$BTC BTC/USDT: This is a very common pair where you are trading Bitcoin (BTC) against Tether (USDT), a stablecoin pegged to the US Dollar. The price of this pair tells you how many USDT you need to buy one BTC.

* BTC/ETH: Here, you are trading Bitcoin against Ethereum. The price indicates how many ETH you can get for one BTC, or vice versa.

* ETH/BTC: This is the inverse of the above. It means you're trading Ethereum against Bitcoin, and the price tells you how much BTC you need to buy one ETH.

* BTC/DOGE, BTC/LTC, BTC/ADA: These are other examples where Bitcoin is paired with various altcoins (alternative cryptocurrencies).

Why are BTC pairs important?

* Gateway to Altcoins: For a long time, and still often, many smaller or newer altcoins can only be traded directly against Bitcoin, not against fiat currencies like USD. So, if you want to buy these altcoins, you often need to first acquire BTC.

* Price Benchmark: Bitcoin is the largest and most established cryptocurrency. Many traders use BTC pairs to gauge the performance of altcoins relative to Bitcoin. If an altcoin's price is rising against BTC, it indicates strength against the overall crypto market.

* Liquidity and Volume: BTC pairs, especially those with major stablecoins or other large cryptocurrencies, tend to have high liquidity and trading volume, making them easier to trade.

In essence, a BTC pair signifies that Bitcoin is involved in the exchange, either as the asset you are selling to acquire another crypto, or the asset you are acquiring by selling another crypto.