How Musical Key Is Stored in Your Music Files

Two places a key can live: the file or the database

Every DJ application keeps a library database: Rekordbox has its collection database, Serato has its _Serato_ folder, Traktor has collection.nml, Engine DJ has its Engine Library database and VirtualDJ has database.xml. When the app analyzes a track, the result lands in that database first. Whether it is also written into the audio file is a separate decision, and the apps disagree about it.

The distinction matters more than it sounds. A key that exists only in a database is tied to one app on one computer. Switch from Serato to Rekordbox, move to a new laptop, or hand a USB stick to a friend on a different setup, and that key is gone — the new app sees an empty field and re-analyzes with its own detector, often producing a different answer. A key stored in the file itself is read by every app that imports it.

MP3 and AIFF: the ID3 TKEY frame

ID3v2 is the tagging format used by MP3 files, and AIFF files carry the same ID3v2 tag inside an ID3 chunk. The ID3v2.3 and ID3v2.4 specifications define a text frame called TKEY, officially titled "Initial key" — the key the track starts in. The older ID3v2.2 format calls the same field TKE. The original ID3v1 tag, the fixed 128-byte block at the end of old MP3s, has no key field at all.

The specification is strict about the value: at most three characters, a note letter A–G, an optional b or #, and an optional m for minor — so "Am", "F#m", "Bb" or "C". A single "o" means the track is off-key (atonal). Reality ignored that rule long ago. Because harmonic mixing tools popularized wheel notations, you will find TKEY values like "8A", "1m", "A minor", "Abm", "G#m" and "08A" in the wild, and DJ software has learned to parse most of them.

Two related habits are worth knowing about. First, some tools — Mixed In Key is the best-known example — can also write the key into the Comment (COMM) field or prepend it to the title, such as "8A - Track Name". That was a workaround for apps and CDJs that did not display TKEY; it still works, but it clutters your titles and duplicates the key in a place nothing validates. Second, Serato stores its own analysis data in private GEOB frames inside the ID3 tag. Those frames are Serato-specific; other apps ignore them, which is why the standard TKEY frame is the one that matters for portability.

FLAC, OGG, M4A and WAV: every format at a glance

Outside ID3, there is no single official key field — only conventions that the DJ and tagging world converged on.

FLAC, OGG Vorbis and Opus use Vorbis comments: free-form NAME=value pairs. The Vorbis comment spec suggests common names like TITLE and ARTIST but nothing for key, so two names coexist. INITIALKEY mirrors the ID3 frame title and is widely used by DJ-oriented tools; KEY is what MusicBrainz Picard and several general-purpose taggers write. Well-behaved software reads both. If you tag by hand, writing the same value to both fields costs nothing and removes the guesswork.

M4A and AAC files use MP4 metadata atoms. Apple never defined a key atom, so the convention is a freeform atom named ----:com.apple.iTunes:initialkey. Apple Music and iTunes do not display it, which surprises people who build playlists there and export to DJ software — but DJ apps and taggers read it. WAV is the awkward one: its native INFO chunk has no key field, so tools embed an ID3 chunk, and whether a given DJ app reads that chunk depends on the app and version.

What goes inside the tag: notation choices

The tag stores text, not a number, so the notation you choose is literally what gets written. All four styles below describe the same key, A minor. The Open Key vs Camelot guide covers which notation is easier to mix with; here the question is which one parses cleanly everywhere.

Watch out for three small inconsistencies that break sorting and filtering:

• Enharmonic spelling. G#m and Abm are the same key (Camelot 1A). If half your library says one and half says the other, a text sort splits them apart. Pick one convention and stick with it.
• Unicode symbols. "F♯m" with the real sharp sign (♯) is not the same string as "F#m" with a hash. Some parsers accept both; others fail silently and show a blank key.
• Zero padding. "08A" sorts correctly as text; "8A" sorts after "12A". Some tools pad to fix sorting, which in turn confuses parsers expecting "8A". Most DJ apps sort by musical key rather than raw text, so padding is rarely needed.

How each DJ app reads and writes key tags

All five major DJ applications read an existing key tag when a track is imported. Where they differ is what happens to their own detection results. Behavior can change between versions, so treat this as the general pattern and check your app's preferences.

The practical upshot: the file tag is the only place all five apps agree to look. If you want one set of keys shown identically across Rekordbox, Serato, Traktor, Engine DJ and VirtualDJ, the key has to be written into the file by a tool you trust — then each app reads it rather than guessing.

Why a new key tag does not show up (and other gotchas)

• The app cached the old value. DJ software reads tags at import and then trusts its database. After re-tagging, use the app's "reload tags" or "read metadata from file" option, or remove and re-import the track. Re-running the app's own key analysis does the opposite of what you want — it replaces the tag value with its own guess.
• Two tools are fighting over the tag. If Serato re-analyzes a folder after another analyzer tagged it, the last writer wins. Decide which tool owns the key, and turn off automatic key analysis (or overwrite) in the others.
• The tag version is too old. Files still carrying only ID3v1 or ID3v2.2 tags may not show a key anywhere. Re-saving the tag as ID3v2.3 — the most compatible version — usually fixes it.
• Apple Music hides it. An M4A with a perfectly valid initialkey atom shows no key in Apple Music. The tag is there; the player simply has no column for it.
• The key is only in the comment. If an old workflow wrote "8A" into Comment instead of TKEY, apps that read TKEY see nothing. Moving the value into the proper key field is covered in how to verify and fix wrong key tags.

How OpenKeyScan writes key tags

OpenKeyScan is built around the file-first approach. It analyzes each track locally with a convolutional neural network (explained in how it works) and writes the result into the file's own metadata: the ID3v2 TKEY frame for MP3 and AIFF, Vorbis comments for FLAC and OGG, and MP4 atoms for M4A. You choose the notation — Open Key, Camelot (alphanumeric) or traditional — so the tag matches whatever your DJ app displays.

Because the key lives in the file, the analysis is done once. Rekordbox, Serato, Traktor, Engine DJ and VirtualDJ all pick it up on import, and a library migration between them no longer means re-detecting and reconciling thousands of keys. It is the same reason Mixed In Key writes to tags — OpenKeyScan just does it for free and offline, and you can download it for Windows and macOS. If you are tagging a large collection for the first time, the batch analysis guide walks through the setup, including backing up first.

Frequently asked questions