Which pairs traded on Forex?

Currency Exchange - a place where the free buying and selling of national currencies, ishV First, it is the world's reserve currency (in brackets are their standard international notation - ISO codes):
U.S. Dollar (USD);
-euro (EUR);
-British pound (GBP);
-Japanese Yen (JPY);
-Swiss franc (CHF).

And other popular currency units:

-Australian dollar (AUD);
-New Zealand dollar (NZD);
-Canadian dollar (CAD);
-Swedish Krona (SEK);
-Norwegian krone (NOK);
-Danish krone (DKK);
-South African Rand (ZAR).

All of these currencies are combined in pairs, and their combination can be anything. Typically, the most widely traded currency pairs 30. Key pairs (accounting for the largest volume of trading) - is the euro / dollar (EUR / USD), pound / dollar (GBP / USD), dollar / yen (USD / JPY) and dollar / Swiss franc (USD / CHF). Much attention is paid to the other pairs involving the U.S. dollar. Cross-rates on the FOREX currency pairs are called, in which the U.S. dollar does not include, for example, EUR / GBP (EUR / GBP), pound / yen (GBP / GPY), pound / Swiss franc (GBP / CHF) and so on.
The Russian ruble is not currently involved in trading on the FOREX, as well as not participating in the free market fluctuations and another major world currencies: Chinese yuan. However, it is likely that the transition to the free convertibility of these currencies will happen in the coming years.