Dominoes spots from one to six are generally arranged as they are on six-sided dice, but because blank ends having no spots are used, seven faces are possible, allowing 28 unique pieces in a double-six set. However, this is a relatively small number especially when playing with more than four people, so many domino sets are "extended" by introducing ends with greater numbers of spots, which increases the number of unique combinations of ends and thus of pieces. Each progressively larger set increases the maximum number of pips on an end by three, so the common extended sets are double-nine, double-12,
Unique user investors. A website's unique users are usually measured over a standard period of time, typically a month. Use of performance indicators such as unique visitors/users is controversial, with Greg Harmon of Belden Research inferring that many companies reporting their online performance "may overstate" the number of unique visitors. Remember, it's just an identifier of a computer, not a person. And usually, the computer is identified by a "cookie" which is most often specific to an individual browser on that computer. Since an increasing percentage of people in the United States (at least) now have access to a computer at home