Double-9 Domino Games
Double-9 domino sets are generally used when more than four players pariticipate in a domino game. Six-player games, in particular, commonly use a X9 domino game set.
The popular game of Chickenfoot Dominoes uses a set of X9 domino tiles. Double 9 dominoes often have different colored spots, which make it easier to distinguish the different point values of a tile when looking across a table.
Chickenfoot and some other Double-9 domino games also may use a center piece to hold the first few dominoes in position. The images below link to a variety of X9 domino sets.