ID3 tags explained: how MP3 files store title, artist and cover art

ID3 tags are the metadata inside an MP3 file: the title, artist, album, track number, genre and cover art that music players, phones and car stereos display. The MP3 standard itself had no place for this information, so ID3 was added on top, first as a small block at the end of the file (ID3v1) and later as a flexible block at the start (ID3v2). The ID3 tag editor reads and rewrites these tags in your browser without touching the audio.

What ID3 tags are

An MP3 file is a sequence of audio frames. Players skip anything that is not audio, which is what makes tags possible: extra bytes before or after the audio are ignored by the decoder and read by the library software. Tags are why one file shows as "Unknown artist" and another shows the full album with artwork. Editing a tag changes only that metadata block; the sound is identical, since nothing is re-encoded.

ID3v1: 128 bytes at the end

The original ID3, from 1996, is a fixed 128-byte block at the very end of the file, starting with the letters TAG. It holds 30 bytes each for title, artist, album and comment, 4 bytes for the year, and 1 byte for a genre chosen from a numbered list, where 0 is Blues and 17 is Rock. ID3v1.1 took two bytes from the comment to store a track number. The limits show quickly: titles longer than 30 characters are cut off, there is no way to store non-Latin scripts such as Greek or Japanese reliably, and there is no room for artwork. Many files still carry a v1 tag alongside a v2 tag for very old players.

ID3v2: frames at the start

ID3v2, introduced in 1998, sits at the start of the file and is made of frames, each with a four-letter ID, a size and its content. The common ones are:

• TIT2 title, TPE1 artist, TALB album, TRCK track number such as 3/12.
• TPE2, officially the band or accompaniment, used by most software as the album artist.
• TYER year in v2.3, replaced by TDRC recording time in v2.4.
• TCON genre, COMM comment, USLT lyrics and APIC attached picture, usually the cover.

Because frames are self-describing, a tag can hold any length of text, multiple images and custom data, and players simply skip frames they do not understand. Tags often include padding, empty space that lets software edit the tag later without rewriting the whole file.

v2.3 or v2.4?

ID3v2.3 (1999) is the most widely supported version. ID3v2.4 (2000) added UTF-8 text, new date frames and some technical changes, but support has always been uneven, and some older players and operating systems show v2.4 tags poorly or not at all. For maximum compatibility, write v2.3, which supports Unicode through UTF-16. That is why the tag editor reads both versions but always saves a fresh v2.3 tag. It also offers to remove an old v1 tag, so a player that prefers v1 does not show stale information. Uncommon frames, such as lyrics and ratings, are not carried over, which is worth knowing before you save over a carefully tagged library.

Cover art and common problems

Cover art is stored in an APIC frame as a JPEG or PNG, marked as the front cover. A square JPEG of about 600 to 1000 pixels is a good balance between sharpness and file size; a huge image can add several megabytes to every track. If a player shows the wrong art, it may be reading a cached copy or a different picture type. Mismatched album artist fields are the usual reason one album splits into several in a library, so set the same album artist on every track. Tags can also carry personal details, such as names in comments, so check them before sharing files, just as What your photos reveal about you recommends for photos.

Other audio formats

ID3 belongs to MP3. M4A and AAC files use MP4 metadata atoms, FLAC and Ogg use Vorbis comments, and WAV has its own chunks, though some software also writes ID3 there. That is why the tag editor accepts MP3 only. If you have a file in another format and want ID3 tags, convert it first with the MP3 converter, bearing in mind that converting from one lossy format to another loses a little quality, as How MP3 compression works, and which bitrate you can actually hear explains. Tags you add to a clip made with the ringtone maker help it show a sensible name in your phone's lists.