Quizzically Yours

X is a silver coin that was minted in the Spanish Empire after a currency reform in 1497. The US dollar was also based upon this coin until the Coinage Act of 1857 discontinued the practice. As it was widely used in Europe, the Americas and the far-East, it became the first world currency by late 18th century. What is the more famous phrase used for X?



 Raghul and Sparky get it right.
Its piece of eight from peso.