$BTC
#btccoinpair Every cryptocurrency has an established value against a fiat currency, while this will continuously change depending on the market conditions affecting it, the use of the same fiat will remain. Most sites default to present a cryptocurrency's value in USD, however, they also allow for changes to be made (to GBP, EUR, etc). This is referred to as a base currency.
A trading pair is when two cryptocurrencies can be traded against each other, as opposed to a cryptocurrency and fiat. For example, BTC/LTC, or ETH/DOGE. In order to acquire the one, the other will need to be exchanged. In this case, the base currency is now a cryptocurrency.
If you would like to buy DOGE with DOT but the exchange does not have this trading pair available, you would need to exchange DOGE for BTC, and then buy DOT with the BTC. Unfortunately, this will incur more fees, illustrating the convenience of trading pairs.