Dominoes domino sets commercially available are double six (with 28 tiles) and double nine (with 55 tiles). Larger sets exist and are popular for games involving several players or for players looking for long domino games. The number of tiles in a set has the formula formula_1 for a double-"n" set. The most popular type of play are layout games, which fall into two main categories, blocking games and scoring games. The most basic domino variant is for two players and requires a double-six set. The 28 tiles are shuffled face down and form the "stock" or "boneyard". Each player draws
Unique identifier Unique identifier With reference to a given (possibly implicit) set of objects, a unique identifier (UID) is any identifier which is guaranteed to be unique among all identifiers used for those objects and for a specific purpose. There are three main types of unique identifiers, each corresponding to a different generation strategy: The above methods can be combined, hierarchically or singly, to create other generation schemes which guarantee uniqueness. In many cases, a single object may have more than one unique identifier, each of which identifies it for a different purpose. In relational databases, certain attributes of an entity that