Proprietary file format

A proprietary file format is a file format of a company, organization, or individual that contains data that is ordered and stored according to a particular encoding-scheme, such that the decoding and interpretation of this stored data is easily accomplished only with particular software or hardware that the company itself has developed. In contrast, a open or free format is a file format that is published and free to be used by everybody.

Some proprietary format may be documented by the developer and released with a note that the format is subject to change without notice, and that the file should only be read or written with libraries provided by the developer. In other cases, the specification of the data encoding format may not be publicly documented at all; in some cases, the format may only be released to those who have signed non-disclosure agreements. A proprietary format can also be a file format whose encoding is in fact published but is restricted through licenses such that only the company itself or licensees may use it.

Proprietary formats are typically controlled by a company or organization for its own benefit, and the restriction of its use by others is ensured through patents or as trade secrets. It is thus intended to give the license holder exclusive control of the technology to the (current or future) exclusion of others.^[1] Typically such restrictions attempt to prevent reverse engineering, though reverse engineering of file formats for the purposes of interoperability is generally regarded as being legal by those who practice it. For example, the US Digital Millenium Copyright Act allows for the reverse-engineering of file formats used for copyright management systems for the purpose of allowing users to exercise their fair use rights to copyrighted media.

As control over a format may be exerted in varying ways and in varying degrees, and documentation of a format may deviate in many different ways from the ideal, there is not necessarily a clear black/white distinction between open and proprietary formats. Nor is there any universally recognized "bright line" separating the two. The lists of prominent formats below illustrate this point, distinguishing "open" (i.e. publicly documented) proprietary formats from "closed" (undocumented) proprietary formats and including a number of cases which are classed by some observers as open and by others as proprietary.