The famous silver 8 Reales coin, known more commonly as the "Spanish Dollar" or "Pieces of 8," created the international standard for silver coinage. So revered for its silver content, the Spanish Dollar still circulated even when it was literally cut up into smaller pieces!
With such universal acceptance, the Spanish Dollar served as the model for U.S. Silver coinage. The 8 Reales denomination was the equivalent of the U.S. Dollar, while the 4 Reales denomination was the equivalent of the U.S. half dollar. Similarly, the 2 Reales denomination, represented a quarter dollar, and it is the reason why the U.S. Quarter is sometimes referred to as "Two Bits," as it represents two pieces of the Pieces of 8.
The Spanish standard also provided for lesser denominations of 1 Reale, and 1/2 Reale, representing 12-1/2 cents and 6-1/4 cents, respectively. Such odd denominations seem very curious today; however, during their time in circulation, many other odd foreign denominations were also in circulation -- such as the British farthing and shilling, and the French franc and ecu. Just imagine the quandary faced by shopkeepers of the day, making change for customers!!