What is Forex?
Foreign Exchange (ForEex) trading is simply the exchanging of one currency for another - Each Forex trade can theoretically be viewed as a 'spread ' trade where to buy one currency you must sell another.

Convention dictates that currencies are measured in units per 1 USD. For example, 1 USD is worth approximately 125 JPY (Japanese Yen) or 1 USD is worth approximately 1.5000 CHF (Swiss Francs).

As a result, when USD/JPY appreciates in value, it is the USD that has appreciated in value relative to the JPY and not vice-versa. Position-wise, to own or be 'Long' USDJPY means that you are long the USD and concurrently short the JPY. USD, therefore, is the default 'lead' currency.

>> Foreign Exchange Market