Publishing companies have existed since the invention of the printing press in the 15th century, when books transitioned from being hand-copied to being mechanically mass-produced. Publishing companies finance the production of books, usually with a high stake in their profits in return. Copyrights, introduced in the 18th century, are usually retained by the company. In the early 21st century, the book market was dominated by large publishers, with most physical book sales produced by five major publishing companies. Independent publishing, an alternative to using a publishing company, became feasible with e-books – which were popularised at the beginning of the 21st century. In 2015-16, there was a rise in physical book sales, coupled with a decrease in e-book sales.