The first currency in a currency pair is called the base Base Currency. It is also known as the transaction currency. The values between the first currency and the second currency in a currency pair is compared in the base currency.
The currency unit prices are quoted as currency pairs in the forex market. The base currency is the first currency which is quoted in a currency pair.
The second part of the currency quotation is called the quote currency or the counter currency.
For example, if you were looking at the EUR/USD currency pair, the euro would be the base currency and the U.S. dollar would be the quote currency.

In forex, currency pairs are written as XXX/YYY or simply XXXYYY.
Here, XXX is the base currency and YYY is the quote currency.
EUR/USD and USD/CAD are two examples of this format.
The abbreviations used for currencies are prescribed by the International Organization for Standardization (ISO). These codes are provided in standard ISO 4217.
Currency pairs use these codes made of three letters to represent a particular currency.
A currency pair in the currencies is usually separated with a slash character. The slash is sometimes removed but it means the same thing.