(from Wikipedia)

While most modern domino pieces are rectangular and made of ceramic or heavy plastic, they were historically carved from ivory or animal bone with small, round pips of inset ebony. The game's name comes from the pieces' resemblance to Venetian Carnival masks known as domini, which were white with black spots. These masks were so named, in turn, because they resembled French priests' winter hoods, being black on the outside and white on the inside. The name ultimately derives from the Latin dominus, meaning "lord" or "master."

The oldest domino sets have been dated from around 1120. Modern dominoes, as most of the Western world knows them, however, appear to be a Chinese invention. They were apparently derived from cubic dice, which had been introduced into China from India some time in the distant past. Each domino originally represented one of the 21 results of throwing two dice. One half of each domino is set with the pips from one die and the other half contains the pips from the second die. Chinese sets also introduce duplicates of some throws and divide the dominoes into two classes: military and civil. Chinese dominoes are also longer than typical European dominoes. Over time Chinese dominoes also evolved into the tile set used to play Mah Jong, a game which swept across the United States in the early to mid 1920s.

The early 18th century witnessed dominoes making their way to Europe, making their first appearance in Italy. The game changed somewhat in the translation from Chinese to the European culture. European domino sets contain neither class distinctions nor the duplicates that went with them. Instead, European sets contain seven additional dominoes with six of these representing the values that result from throwing a single die with the other half of the tile left blank. Curiously, there is also a seventh tile with both halves set as blank. Perhaps this was done for symmetry's sake so that each of the resulting suits would contain seven dominoes each.

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• Double-Six Domino Set:
  • 28 Domino Tiles (6:6 to Blank:Blank)
• Double-Nine Domino Set:
  • 55 Domino Tiles (9:9 to Blank:Blank)
• Double-Twelve Domino Set:
  • 91 Domino Tiles (12:12 to Blank:Blank)
• Double-Fifteen Domino Set:
  • 136 Domino Tiles (15:15 to Blank:Blank)