MOBI

MOBI. MOBI is a e-book file format originally developed by the French company Mobipocket SA. Based on the Open eBook standard using XHTML, it can include JavaScript and frames, and is designed for use on mobile devices with limited memory. The format was notably adopted and later superseded by Amazon Kindle's proprietary formats, cementing its role in the early history of digital publishing.

Overview

The MOBI format was created to deliver reflowable digital content to early personal digital assistants and smartphones, leveraging the existing Open eBook publication structure. It gained significant traction after its acquisition by Amazon.com in 2005, which used it as the foundation for its Amazon Kindle ecosystem. While largely succeeded by formats like AZW3 and KFX, MOBI files remain widely distributed through platforms like Project Gutenberg and are supported by numerous third-party e-reader applications such as Calibre and FBReader. Its design prioritized compression and efficient resource handling, making it suitable for the hardware constraints of devices from companies like Palm, Inc. and Microsoft.

Technical specifications

A MOBI file is essentially a modified Palm Database file (with a .prc extension) that wraps XHTML or HTML content, along with embedded resources like JPEG images. It supports a subset of CSS for styling and can include a built-in dictionary for on-the-fly word lookups. The format utilizes Lempel–Ziv–Welch compression within its PalmDOC component to minimize file size. Unlike later EPUB standards, MOBI files are typically packaged as a single file, with metadata stored in a dedicated Palm OS record. The format's structure was directly evolved into Amazon's AZW format, which added DRM through a system called Amazon's Digital Rights Management.

Applications and use cases

Primarily, MOBI files were used for distributing commercial and free e-books, with early adopters including online retailers and digital libraries. The format was the default for millions of Amazon Kindle devices sold prior to the shift toward EPUB support, and it remains a common export option from writing software like Scrivener. Academic institutions and public libraries utilized the format for distributing texts through services like OverDrive. Furthermore, its support for JavaScript allowed for simple interactive content, making it a tool for creating digital manuals and guides for software from companies like Adobe Systems and Microsoft.

Development and history

The format was developed by Mobipocket SA, founded in 2000, which released its first e-reader software for the Palm OS platform. Key to its early growth was support on devices from Symbian and the Windows Mobile operating systems. Amazon.com's acquisition of Mobipocket in 2005 was a strategic move ahead of the launch of the first Amazon Kindle in 2007. For years, Amazon provided a Mobipocket Creator tool for authors and publishers. However, development stagnated after the acquisition, and Amazon officially deprecated the format in 2022, ending support for direct MOBI email-to-Kindle delivery in favor of EPUB.

Standards and compatibility

While MOBI was based on the open Open eBook standard, it became a de facto proprietary format under Amazon. It is not compliant with the modern, widely adopted IDPF EPUB standard. Despite this, compatibility remains broad due to extensive third-party support; conversion tools within Calibre can transform MOBI files into formats like PDF, EPUB, and AZW3. The format's variants, such as the DRM-free AZW4, illustrate its evolution within the Amazon ecosystem. Its legacy persists in the underlying structure of many Amazon Kindle files, even as the industry shifts toward the standardized EPUB 3 specification.

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