MOBI (file format)

MOBI (file format)

The MOBI file format is a digital book format originally developed by the French company Mobipocket for use on personal digital assistants and early smartphones. It is based on the Open eBook standard using XHTML and can include JavaScript and frames. The format was later acquired by Amazon.com and formed the foundation for its proprietary Kindle formats, though it has since been largely superseded.

History and development

The format was created in 2000 by Mobipocket, a company founded by employees of the French firm CIC bank. Its development was influenced by the Open eBook Publication Structure and aimed to provide a feature-rich format for the burgeoning Palm OS and Windows CE device markets. In 2005, Amazon.com purchased Mobipocket, primarily acquiring its technology and patents. Amazon used the MOBI structure as the core for its new AZW format, which debuted with the first Amazon Kindle in 2007. For several years, Amazon accepted submissions in the legacy MOBI format through its Kindle Direct Publishing platform, but officially deprecated it in favor of EPUB in 2022.

Technical specifications

A MOBI file is essentially a modified Palm Database (PDB) file, a container format historically used by Palm, Inc. devices. Internally, it wraps XHTML content, along with a binary PalmDoc compression layer and a proprietary HUFF/CDIC compression scheme for textual data. The format supports embedded JavaScript, SQLite databases for annotations, and a complex record structure for indexing and DRM. Metadata is stored using the EXTH header, which allows for extensive tagging beyond the standard Palm OS database headers. Unlike modern formats, its handling of CSS and complex page layouts is generally limited.

Usage and compatibility

The primary historical usage was for distributing e-books to devices like the Palm Pilot, Psion organizers, and early BlackBerry smartphones. Following the acquisition by Amazon.com, its main utility shifted to being a source format for conversion to Kindle-compatible AZW files. While modern Kindle devices and apps no longer natively read the original MOBI format, they support its direct successors. The format remains in limited use within certain Calibre library ecosystems and among archivists of early digital publishing. Its compatibility with modern EPUB-centric reading systems is virtually nonexistent without prior conversion.

Comparison to other formats

Compared to the open standard EPUB, the MOBI format is less flexible in rendering complex layouts and typography, as it was designed for the limited screens of early Palm OS hardware. Unlike PDF, it features reflowable text but lacks precise pagination and design fidelity. Its successor, the AZW format, added a more robust DRM scheme and better integration with the Amazon Kindle Store. The KFX format, a later evolution from Amazon.com, further advanced typography and layout features beyond the original MOBI foundation. In contrast to Apple's proprietary iBooks Author format, MOBI was never intended for highly interactive, multimedia-rich publications.

Software support

Original support was provided by the Mobipocket Reader application on platforms including Windows, Palm OS, and Symbian. The most significant contemporary software supporting the format is the open-source Calibre e-book management suite, which can read, convert, and catalog MOBI files. Other legacy readers like FBReader and STDU Viewer have offered varying levels of support. Modern e-reader devices from Kobo, Barnes & Noble (Nook), and Sony do not support the format natively, focusing instead on EPUB and PDF. Amazon's own Kindle Previewer tool historically accepted MOBI for conversion but now recommends EPUB.

Category:Amazon (company) Category:E-book formats Category:Computer file formats