Despite the difference in currencies, the monetary system was different in a key aspect: the coins were made of gold and other valuable metals.

Which means that if you make a trade for X pounds of silver coins, it will be worth the same to you regardless of what is printed in the coins since silver could be trade anywhere.

Today, most countries currencies no longer hold real value in Gold or Silver, therefore their value is directly linked to other economical factors, such as the amount of the currency has been printed and is in circulation.

EDIT: grammar.