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MP3 sound files may contain metadata that describes the content. The APE tag is one of the forms of metadata. It follows the sound data at the end of the file, unlike the common ID3 tags that precede the sound data. This makes expansion easier. APE field names ("keys") are character strings that may or may not follow a standard, unlike the ID3 tag "frames" that are identified by standardized 4 byte codes.
APE tags are more similar to Vorbis comments than ID3 tags. Like Vorbis comments, they are unstructured (key,value) pairs. However, unlike Vorbis comments, they do not allow for inter-key ordering. This is because they store a list of values for each key rather than one value per key.
APE values can be flagged as text, binary, or external types. This allows tag editing software to avoid incorrectly displaying binary values, such as an image of album cover art, in the form of unreadable text to users. In comparison, Vorbis comments do not have a flagging feature, so binary data cannot be easily stored in them (though this is by design).
The APEv1 tag was designed for the Monkey's Audio format. In MP3 files, the APE tag is stored at the very end of the file, with no inline declaration in the body of the file. The software handles the writing and access to the tag and does not interfere with the contents of the MP3.
The Musepack format developer, Frank Klemm, extended the original APE tag format to add a header, allowing APE tags to be at the beginning of files and allowing metadata values to be Unicode rather than simply ASCII. Because of its simplicity and flexibility, APEv2 was adopted by the WavPack and OptimFROG formats as their primary tag format. Version 3.99 of the official Monkey's Audio software switched from using APEv1 to APEv2.
The popular media player software, Winamp, supports reading and writing of APEv2 tags in MP3 files. foobar2000 and Jack! The Knife can tag MP3 files with APEv2 tags instead of ID3 tags. Some music library managers and mass taggers such as MusicBee or Mp3tag also support this feature. The tagging string APETAGEX signals the start of an APEv2 record, and the string TAG signals the start of an ID3v1 tag.