The "long tail theory" believes that all non-mainstream markets will add up to a market larger than the mainstream market. These small demands will form a long "tail" on the demand curve, achieving the maximum of the niche. Quantity. On the long "tail", the "personalization" that has been squeezed and ignored by the popular will be highlighted. A typical example is an opportunity that the Internet brings to books. A book is in a bookstore and online The production and dissemination costs are not the same. The unlimited space of the network reduces costs, non-mainstream, and the total sales volume with the smallest demand can always match the total volume of the mainstream and best-selling best-sellers. The following description is contextual. ['The essence and content of the "Long Tail Theory".', 'A large number of demand in the niche market can also match the mainstream market.', "The Internet's Opportunities for Books.", 'Non-mainstream, niche with the least demand will replace the mainstream, the most demanding popular.']