The currency pairs of forex market can be classified into three categories as below:
Direct currency
Direct currency is the currency which exchanged with USD, e.g. GBP/USD, EUR/USD and another forex pair that compared with USD and the USD written at the behind
Indirect currency
Indirect currency is the currency that exchanged to buy USD e.g. USD/JPY, USD/CHF and another forex pair with compared with USD and the USD written at the front
Cross Rate
Cross rate is the currency which not compared with USD e.g. GBP/JPY, EUR/CHF, AUD/CAD etc.
