Your question implies that you understand the concept of buying a currency pair, and then making a profit when the price rises and you later sell it.
Buying a currency pair in this way is called “going LONG”, or “taking a LONG position”. You made a profit because you bought low and sold high.
SELLING SHORT is just the opposite. You open a new position by selling first; then, later, you close your position by buying.
If the price of the currency pair declines after you sell short, then when you buy it back, you make a profit, because you sold high and bought low.
In the examples, above, a currency pair was treated as a single “thing” which you can buy or sell. But, a currency pair is obviously a combination of two currencies, as well; and its value is determined by the relative values of those currencies.
Whenever something affects the value of an [B]individual currency[/B], we can easily determine how that will affect the value of a [B]currency pair [/B]involving that currency.
Take the GBP as an example. Suppose some bad news comes out, negatively affecting the pound (GBP). We know immediately that this news will tend to push the GBP/USD [B]down[/B], and it will tend to push the EUR/GBP [B]up[/B].
As a general rule:
[B]If the value of the base currency rises, the value of the currency pair rises.
If the value of the base currency falls, the value of the currency pair falls.
If the value of the cross currency rises, the value of the currency pair falls.
If the value of the cross currency falls, the value of the currency pair rises.
[/B]
Make sure you understand why these statements are true.
I hope this answers your questions.
Clint