How the chord identifier names a chord
Pick the notes, and the identifier lists every chord those notes can spell, with the most likely reading first. It tries each note you played as a possible root, measures the intervals of the other notes above it, and matches the result against 33 chord shapes, from power chords and triads to thirteenths and altered dominants.
The lowest note decides the inversion. When it is the root, the chord is in root position. When it is another chord tone, the symbol gets a slash and the bass note, so C major with E underneath is written C/E.
Octave doublings do not change the name, so a chord spread across four octaves reads the same as one in close position. Typed notes without octave numbers are stacked in the order you type them, which makes the first note the bass.
Why the same notes can have two names
A, C, E and G make A minor seventh. Put the C at the bottom and the same four notes read more naturally as C major sixth.
Nothing in the notes says which is right. The bass line and the harmony around the chord decide, which is why the identifier keeps the second reading in view instead of hiding it.
Symmetrical chords are the extreme case. A diminished seventh divides the octave into four equal minor thirds, so C, E♭, G♭ and A can be named from any of its four notes.
An augmented triad splits the octave into three major thirds and has three equally valid roots. All of them are listed, with the reading built on the bass first.
The spelling follows the chord, not the key you happened to press. A diminished seventh on C is written with B𝄫 rather than A, because its top note is a seventh above the root. Copy that spelling into a score and it reads correctly to a player.
Inversions and slash chords
First inversion puts the third in the bass, second inversion the fifth, and third inversion the seventh. Anything else in the bass, such as the ninth, a sixth or a note outside the chord, is shown as a slash chord with that note after the slash.
On a lead sheet or a sketch, slash chords tell an orchestrator what the bass does without writing out a separate line. In a mockup they are a reminder that the low strings and the piano's left hand shape the harmony instead of only doubling it.
Extended chords and missing fifths
Ninths, elevenths and thirteenths are rarely voiced with every note. The fifth is the first note arrangers drop, because it adds weight without changing the chord's quality. The eleventh is usually left out of a dominant thirteenth because it clashes with the third.
The identifier accepts those omissions and says when a reading depends on one. When no shape accounts for every note, it offers the nearest readings and names the note left over, so C, E, F♯ and G come back as C major with an added F♯. Treat that as a prompt to look at the voicing again, not a verdict on it.
Using it while you write
It is quickest for checking harmony you did not write yourself. Play a chord from a pad or phrase patch whose notes are not documented, transcribe a few chords from a temp track to see what the director is attached to, or check a voicing spread across a string section before orchestrating the rest of the cue.
Connect a MIDI keyboard to read chords as you play them. Held notes form the chord, the sustain pedal holds them, and the last chord stays on screen after you let go. Most composers sketch harmony on a piano patch first, and our roundups of the best piano VSTs for composers and the best cinematic pianos cover the instruments built for it.