The American Book Number (ISBN) is an international standard book number, which is used to uniquely identify a book. Publishers usually apply for and obtain a unique ISBN for books published in the United States. This ISBN can help bookstores, libraries and other sales channels accurately identify and track books. In the United States, bookstores are one of the main retail channels for books. When the ISBN of a book has been registered and associated with the publisher, the book can be sold in the United States through various channels, including independent bookstores, chain bookstores and online bookstores. Bookstores will purchase books from publishers or wholesalers and put them on bookshelves for consumers to buy.