A chord is a group of notes played together to create harmony. Understanding how chords are built — which intervals they use, what each note’s role is, and how they relate to one another — is one of the most practical skills in music, whether you compose, arrange, improvise, or simply want to understand what you are hearing. This tool lets you both build any chord from scratch and identify a chord by clicking its notes.
How it works
Every chord is defined relative to a root note by a list of semitone intervals. A semitone is the smallest step in Western equal-temperament tuning — one piano key, one guitar fret. The major triad uses offsets (0, 4, 7): root, a major 3rd (4 semitones above root), and a perfect 5th (7 semitones above root). Sharpen the 3rd to 4 and you get a major chord; flatten it to 3 and you get a minor chord. Flatten the 5th to 6 and the 3rd stays minor — that is a diminished chord.
For note frequencies, the tool uses the standard equal-temperament formula:
f = 440 × 2^((n − 69) / 12)
where n is the MIDI note number (middle C = 60, A4 = 69 = 440 Hz). Each semitone multiplies the frequency by the 12th root of 2, approximately 1.0595. That means the notes of a C major chord at octave 4 ring at roughly 261.6 Hz (C4), 329.6 Hz (E4), and 392.0 Hz (G4).
In Identify mode the logic is reversed: the tool tests all 12 possible roots against all 24 chord types and returns every combination whose interval set exactly matches the notes you selected — useful when you hear a chord and want to know its name.
Worked example — G dominant 7th
Choose root G and chord type Dominant 7th. The interval offsets are [0, 4, 7, 10]:
| Semitones from G | Interval | Note | Freq (oct 4) |
|---|---|---|---|
| 0 | Root | G | 392.0 Hz |
| 4 | Major 3rd | B | 493.9 Hz |
| 7 | Perfect 5th | D | 293.7 Hz |
| 10 | Minor 7th | F | 349.2 Hz |
The minor 7th gives G7 its characteristic tension that wants to resolve to C major — the foundation of the dominant-to-tonic motion in Western harmony.
Formula note
All 24 chord types in the library are derived from the same principle: choose a root, apply a fixed set of semitone offsets, and reduce each result modulo 12 to stay within one octave. Extended chords (9th, 11th, 13th) use offsets beyond 12, which the frequency calculator maps to the correct higher octave. Enharmonic spelling (C# vs Db, F# vs Gb) does not change the pitch — both names refer to the same physical frequency; the tool shows the sharp spelling internally and notes the flat equivalent where it exists.