JAM is both the software and file format for representing music as human-readable and human-writable text. Unlike the ABC notation, another text-based music format, that is best suitable for one-voice tunes, JAM is mainly focused on chords.
Here is an example of jam notation: ### LULLABY OF BIRDLAND
Dm7 Bm7-5 | E7 A7 | Dm7 / | Gm7 C7
F+7 F7 | Bb+7 Bbm7 | F+7 / | Em7-5 A7
Dm7 Bm7-5 | E7 A7 | Dm7 / | Gm7 C7
F+7 F7 | Bb+7 Bbm7 | F+7 C7 | F+7 /
Am7-5 D7 | Gm7 / | Gm7-5 C7 | F+7 /
Am7-5 D7 | Gm7 / | Gm7-5 C7 | F+7 A7
Dm7 Bm7-5 | E7 A7 | Dm7 / | Gm7 C7
F+7 F7 | Bb+7 Bbm7 | F+7 C7 | F+7
The software is proprietary, windows-only freeware.