EPUB (short for electronic publication; alternatively capitalized as ePub, EPub, or epub, with "EPUB" preferred by the vendor) is a free and open e-book standard by the International Digital Publishing Forum (IDPF). Files have the extension .epub. EPUB is designed for reflowable content, meaning that the text display can be optimized for the particular display device used by the reader of the EPUB-formatted book. The format is meant to function as a single format that publishers and conversion houses can use in-house, as well as for distribution and sale. It supersedes the Open eBook standard.