The "Drop A/C" tuning for a 7-string guitar is a profoundly low and heavy setup, designed to deliver maximum impact and depth. As its name implies, it's built upon a foundation of Drop C, but extended even further downwards with a thunderous low A on the 7th string. This configuration is a staple for modern metal genres, particularly djent, deathcore, and other styles that demand a crushing, resonant sound.

Technically, the open strings are tuned to A1, C2, G2, C3, F3, A3, D4, from the lowest (7th) to the highest (1st) string. This creates a very wide tonal range, spanning nearly four octaves. The low A1 provides an incredibly deep foundation, while the C2-G2-C3 segment (strings 6, 5, 4) forms a robust C power chord, characteristic of "Drop" tunings and perfect for chunky, single-finger power riffs. The subsequent intervals of a perfect fourth (G2 to C3), perfect fourth (C3 to F3), major third (F3 to A3), and perfect fourth (A3 to D4) on the higher strings offer a blend of standard and non-standard relationships, opening up unique harmonic possibilities.

While not designed for standard open chords, this tuning excels at powerful, dissonant, and extended voicings. The open 6th, 5th, and 4th strings (C-G-C) instantly provide a massive C power chord. Playing the lowest three strings open (A1-C2-G2) yields an A minor sound (root, minor third, perfect fifth, with omitted E), or a C major chord with an A bass, offering a dark and complex resonance ideal for atmospheric intros or heavy breakdowns. Due to the altered intervals, traditional barre chord shapes will need adaptation, but the consistent perfect fourths on strings 5-4, 4-3, and 2-1 facilitate easy power chords and modal playing on those groups. The major third between strings 3 and 2 (F3-A3) can be leveraged for interesting major triad or dominant 7th voicings.

How to Tune:

To achieve the "Drop A/C" tuning, carefully adjust each string as follows. It's recommended to start from a standard 7-string tuning (such as B-E-A-D-G-B-E), but the movements below specify the amount each string needs to be lowered to reach its target note. Always use a reliable tuner and ensure your guitar's intonation is properly set after making significant tuning changes.

• 7th String (A1): Tune down 2 semitones to reach A1.
• 6th String (C2): Tune down 4 semitones to reach C2. Please note: tuning down 4 semitones is a significant drop. If your guitar is not typically set up for such low tunings, you may experience string buzzing or intonation issues. A heavier gauge string for this position is strongly advised to maintain optimal playability and tone.
• 5th String (G2): Tune down 2 semitones to reach G2.
• 4th String (C3): Tune down 2 semitones to reach C3.
• 3rd String (F3): Tune down 2 semitones to reach F3.
• 2nd String (A3): Tune down 2 semitones to reach A3.
• 1st String (D4): Tune down 2 semitones to reach D4.

Given the extensive downtuning, especially on the 6th string, a setup with heavier gauge strings is highly recommended across the board to ensure proper tension, intonation, and clarity, especially on the lower strings. This tuning opens up a world of heavy, percussive riffing and dark harmonic exploration.