ABC Tools

How Chord Naming Works

After reading this you will be able to name any chord from its notes, understand why the same four pitches can carry two different names, and read the ranked list of candidates the tool produces.

What the tool does and one example

A chord is a set of pitches heard together. Naming it means finding a root note and describing the other notes as intervals above that root. The Chord Identifier takes the notes you pick, tries each note as a possible root, measures the intervals, and matches those intervals against a dictionary of known chord shapes.

Play the four notes C, E, G, and A together. The tool reports two strong candidates: C6 (a C major triad with an added sixth) and Am7 (an A minor seventh in an inverted position). Both names describe the exact same four pitches. That ambiguity is not a bug. It is a real property of the twelve-tone system, and the tool exposes it rather than hiding it.

When to use it, and when not to

Use it when you have notes and want a name: you sight-read a cluster off a lead sheet, you record a chord on guitar and want to write it down, or you are learning how a jazz voicing is built. It answers "what is this?" quickly and shows the intervals so you can learn the pattern.

Do not use it to decide which name is correct in a musical context. Correctness depends on the key and the surrounding chords, and the tool has no key context. If you handed it the notes C, E, and G, it knows those spell a C major triad, but it cannot know whether the piece is in C major or in G major where the same chord is the subdominant. For key and scale questions, reach for the Scale Finder instead.

All the pitch math assumes twelve-tone equal temperament, the tuning of pianos, guitars and digital keyboards. In that system every semitone is exactly the same size, so F# and Gb are the same key. The tool treats them as identical and picks a spelling by convention.

The interval formula

Give each note a pitch class number from 0 to 11: C=0, C#=1, D=2, and so on up to B=11. The interval of a note above a chosen root is the difference measured on a 12-hour clock.

i = (p - r) \bmod 12

Here p is the pitch class of the note, r is the pitch class of the candidate root, and i is the interval in semitones, always reduced to the range 0 to 11. The tool computes the full set of intervals for each candidate root, sorts them, and compares that set to every entry in its dictionary.

A major triad, for example, is stored as the interval set \{0, 4, 7\}: the root, a major third (4 semitones), and a perfect fifth (7 semitones). A minor triad is \{0, 3, 7\}. The only difference is the third: 4 semitones versus 3. That one semitone is the whole distinction between happy and sad in Western harmony.

The intervals used most often are these:

Common chord tones as semitone intervals above the root
Interval nameSemitonesAppears in
Minor third3minor, diminished
Major third4major, augmented, dominant
Perfect fifth7most triads
Major sixth9sixth chords
Minor seventh10dominant and minor sevenths
Major seventh11major seventh chords

Inversions and why the root is not the lowest note

The root is the note the chord is built from, not necessarily the note played lowest. When the lowest note is something other than the root, the chord is inverted. To find the root, the tool rotates through every note as a candidate and keeps the rotation whose intervals match a named shape most cleanly.

Take E, G, C played bottom to top. If you assume E is the root, the intervals are \{0, 3, 8\}, which matches no simple triad. Try C as the root and you get \{0, 4, 7\}, a clean C major triad. So the tool reports C major in first inversion, with E in the bass. The bass note is described separately as a slash chord, written C/E.

If you want the tool to commit to a specific root, put that root as the lowest note. When two rotations tie, a root in the bass usually breaks the tie in favor of the name you intended.

Worked example: C, E, G, A

Reproducing the demo

Enter the four default notes C E G A and read the intervals for each candidate root.

  1. Assign pitch classes: C=0, E=4, G=7, A=9.
  2. Try C as root (r=0). Intervals: \{0, 4, 7, 9\}. That is root, major third, perfect fifth, major sixth. It matches the dictionary entry for a sixth chord, so the name is C6.
  3. Try A as root (r=9). Compute each: C gives (0-9)\bmod 12 = 3, E gives (4-9)\bmod 12 = 7, G gives (7-9)\bmod 12 = 10, A gives 0. Intervals: \{0, 3, 7, 10\}, which is root, minor third, perfect fifth, minor seventh: a minor seventh chord, Am7.
  4. The two candidates use identical pitches, so both are reported. If C is lowest, C6 ranks first; if A is lowest, Am7 ranks first.
The same four pitches produce interval set {0,4,7,9} from C and {0,3,7,10} from A. Two roots, two valid names.

Reading the ranked candidate list

The tool lists every match, ranked by fit. Ranking rewards two things: a root that sits in the bass, and a shape that uses no leftover or missing tones. A four-note set that fills a named four-note shape exactly ranks above a four-note set read as a five-note chord with one note omitted.

Root
The note the intervals are measured from. Shown for each candidate, not just the top one.
Inversion
Which chord tone is in the bass. Root position means the root is lowest; first inversion puts the third lowest; second inversion puts the fifth lowest.
Intervals above the root
The semitone list the name was matched from. Use this to check the tool's reasoning by hand.
Candidate
A plausible name. More than one is normal. The order reflects fit, not musical truth.

A C triad with the third fixed as C and the fifth as G. Moving the middle note from E flat (3 semitones) to E natural (4 semitones) switches the name from C minor to C major; lowering the top note from G to G flat (6 semitones) with a minor third makes C diminished; raising it to G sharp (8 semitones) with a major third makes C augmented.

Common mistakes

Watch for these traps when you read a result.

  • Reading the bass as the root. The lowest note is often not the root. E G C is C major, not an E chord.
  • Expecting one true name. The C6 and Am7 case is not rare. Many four-note sets are shared. The tool lists all of them because they are all correct as pure note collections.
  • Confusing enharmonic spellings. The tool may print F# where your sheet has Gb. They are the same key on a piano. The spelling follows a sharp or flat convention, not the source you copied from.
  • Entering fewer than three notes. Two notes form an interval, not a chord. A single note has no quality. Power chords (root plus fifth, \{0, 7\}) are the one two-note case the dictionary names.

Do not read the top-ranked candidate as the musically correct one. Ranking measures how tidily the notes fill a shape, not how the chord functions in a key. A C6 in a jazz tune and an Am7 in a pop ballad can be the identical four notes; only the surrounding music decides which name belongs.

Related tools

Once you know a chord's name, you can ask what scales contain it or how it moves to the next chord. The Scale Finder takes the same notes and lists every scale and key they fit, which is the fastest way to resolve the naming ambiguity that this tool leaves open. When you want to string chords together and hear them, the Chord Progression Builder lets you build and randomize progressions in any key. For rhythm rather than harmony, the Euclidean Rhythm Generator spreads a number of hits evenly across a number of steps.

Frequently asked questions

Why does my chord have two names?

Because the notes fit two shapes equally. C6 and Am7 both contain C, E, G, and A. Neither name is wrong. Choose the one that matches your key: A minor seventh in A minor, C sixth in C major.

How many notes do I need for a chord?

Three for most named chords. Two notes make an interval, and the only two-note name in the dictionary is the power chord (root and fifth, 7 semitones apart). One note has no chord quality at all.

Why is the lowest note not always the root?

Because chords can be inverted. The root is defined by the interval pattern, not by which note sits at the bottom. When the lowest note differs from the root, the tool reports an inversion and a slash name like C/E.

Does it handle sharps and flats the same way?

Yes. In equal temperament F# and Gb are the same pitch, so the tool treats them as one note and prints whichever spelling fits the usual sharp or flat convention.

Can it name a chord with a wrong or missing note?

It matches complete shapes best. If one note is missing, it may still find a name by treating your notes as a partial voicing, but that candidate ranks below any exact match. Add or remove notes and watch the ranking change.

Try it yourself in the tool

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