Market forces

If a currency is free-floating, its exchange rate is allowed to vary against that of other currencies and is determined by the market forces of supply and demand. Exchange rates for such currencies are likely to change almost constantly as quoted on financi market.

offering sopt exchange
In 2005 Barclays Capital broke with convention by offering spot exchange rates with 5 or 6 decimal places on their electronic dealing platform[2]. The contraction of spreads (the difference between the bid and offer rates) arguably necessitated finer pricing and gave the banks the ability to try and win transaction on multibank trading platforms where all banks may otherwise have been quoting the same price. A number of other banks have now followed this

Market convention from the early 1980s to 2006 was that most currency pairs were quoted to 4 decimal places for spot transactions and up to 6 decimal places for forward outrights or swaps. (The fourth decimal place is usually referred to as a "pip"). An exception to this was exchange rates with a value of less than 1.000 which were usually quoted to 5 or 6 decimal places. Although there is no fixed rule, exchange rates with a value greater than around 20 were usually quoted to 3 decimal places and currencies with a value greater than 80 were quoted to 2 decimal places. Currencies over 5000 were usually quoted with no decimal places (e.g. the former Turkish Lira). e.g. (GBPOMR : 0.765432 - EURUSD : 1.5877 - GBPBEF : 58.234 - EURJPY : 165.29). In other words, quotes are given with 5 digits. Where rates are below 1, quotes frequently include 5 decimal places

Market convention
There is a market convention that determines which is the base currency and which is the term currency. In most parts of the world, the order is: EUR – GBP – AUD – NZD – USD – others. Thus if you are doing a conversion from EUR into AUD, EUR is the base currency, AUD is the term currency and the exchange rate tells you how many Australian dollars you would pay or receive for 1 euro. Cyprus and Malta which were quoted as the base to the USD and others were recently removed from this list when they joined the euro. In some areas of Europe and in the non-professional market in the UK, EUR and GBP are reversed so that GBP is quoted as the base currency to the euro. In order to determine which is the base currency where both currencies are not listed (i.e. both are "other"), market convention is to use the base currency which gives an exchange rate greater than 1.000. This avoids rounding issues and exchange rates being quoted to more than 4 decimal places. There are some exceptions to this rule e.g. the Japanese often quote their currency as the base to other currencies.

Exchange system
An exchange system quotation is given by stating the number of units of "term currency" (or "price currency" or "quote currency") that can be bought in terms of 1 "unit currency" (also called "base currency"). For example, in a quotation that says the EURUSD exchange rate is 1.4320 (1.4320 USD per EUR), the term currency is USD and the base currency is EUR.

Exchange rates
In finance, the exchange rates (also known as the foreign-exchange rate, forex rate or FX rate) between two currencies specifies how much one currency is worth in terms of the other. It is the value of a foreign nation’s currency in terms of the home nation’s currency.[1] For example an exchange rate of 95 Japanese yen (JPY, ¥) to the United States dollar (USD, $) means that JPY 95 is worth the same as USD 1. The foreign exchange market is one of the largest markets in the world. By some estimates, about 3.2 trillion USD worth of currency changes hands every day.
The spot exchange rate refers to the current exchange rate. The forward exchange rate refers to an exchange rate that is quoted and traded today but for delivery and payment on a specific future date

Recently
Trading in the euro has grown considerably since the currency's creation in January 1999, and how long the foreign exchange market will remain dollar-centered is open to debate. Until recently, trading the euro versus a non-European currency ZZZ would have usually involved two trades: EUR/USD and USD/ZZZ. The exception to this is EUR/JPY, which is an established traded currency pair in the interbank spot market. As the dollar's value has eroded during 2008, interest in using the euro as reference currency for prices in commodities (such as oil), as well as a larger component of foreign reserves by banks, has increased dramatically. Transactions in the currencies of commodity-producing countries, such as AUD, NZD, CAD, have also increased

Positive currency
The factors affecting XXX will affect both XXX/YYY and XXX/ZZZ. This causes positive currency correlation between XXX/YYY and XXX/ZZZ.
On the spot market, according to the BIS study, the most heavily traded products were:
EUR/USD: 27%
USD/JPY: 13%
GBP/USD (also called sterling or cable): 12%
and the US currency was involved in 86.3% of transactions, followed by the euro (37.0%), the yen (17.0%), and sterling (15.0%) (see table). Note that volume percentages should add up to 200%: 100% for all the sellers and 100% for all the buyers