A magazine's circulation is the number of copies it usually distributes for each issue. It is one of the main factors used to determine how much money they will charge businesses to advertise in the magazine. Circulation is not always the same as copies sold, which is usually called paid circulation, because many magazines are distributed to readers without making them pay anything. This is more true for magazines dealing with business and professional topics. When determining readership figures, which is how many people read an issue, the number is usually considered to be higher than the circulation figures because of the assumption that most copies of the magazine are read by more than one person. In many countries, circulation figures are checked by companies that do not work for the magazine, such as the Audit Bureau of Circulations, to make sure that the numbers a publisher gives to advertisers is accurate.