During the Golden Age of Piracy (and well into the 19th Century) A Piece of Eight was a Spanish or Spanish American coin roughly that was roughly equivalent to today's dollar coin. However, unlike today's American dollar which is worth 100 pennies, the Piece of Eight was worth, you guessed it, eight of Spain's small common denomination, the Reale (sometimes spelled "real").
As such the Piece of Eight was clearly marked with the number "8". It may sound strange to some people to have 1/8 pieces but at one time the U.S. Dollar also was divided into eight pieces or bits. Remember the nursery rhyme "two bits, four bits, six bits, a dollar"?
Today, we would call it an "Eight Real Piece or Coin" similar to a "$20 Gold Piece" or "Ten Mark Piece" Just about any reale might be referred incorrectly as a "Piece of Eight" but all where clearly minted as 1, 2, 4, and 8 reale pieces. Sometimes, the coins would be cut up or cut in half, so if you cut an 8 reale coin in half the two halves would each be worth four reales. Because the coins were made of pure silver, cutting them into pieces did little to decrease their value.