I had no idea the game had so many variations. Double Six, Double Nine, Double Twelve, and Double Fifteen stood out as separate sets of dominoes. Did you know there's even a Pro Domino Association? There have even been games televised on ESPN. Who knew? (It must have been at night when I was sleeping, because I stay pretty in-tune with what's airing on ESPN and I definitely missed the dominoes.
The PDA hosts tournaments throughout the country and promotes the sport to increase interest in the game. From PDADominoes.com :
The PDA has established a leadership brand in scope with other recognizable professional sport and game authorities. We have created a competition infrastructure and maintain stewardship over the game in the worldwide community of domino players.