MusicReasonChords and Harmony
What is this chord doing in this key?
Enter a chord, a key, and optionally the chord that follows it. This tool explains the chord's likely harmonic function, and shows alternatives rather than a false unique answer when the evidence does not establish one.
Answer
Want to learn more? Chord Function, Cadence, and Substitution
Understand
The same chord can serve different functions depending entirely on what surrounds it: a D major chord in the key of C is a plain diatonic ii-adjacent chord when built as D minor, but reads as an applied dominant (V/V) when built as D7 and followed by G. This tool never asserts a unique function without that surrounding evidence.
Worked examples
G7 in the key of C major presents as V7, the diatonic dominant seventh. D7 immediately before a G chord in C major presents as V7/V, an applied dominant preparing the dominant. A-flat major in the key of C major is not diatonic at all; it permits a modal-mixture explanation, borrowed from C minor's submediant.
Method and limitations
The tool spells the declared key's diatonic scale, compares the chord's root and quality to each scale degree's expected triad or seventh, and checks a following chord for an applied-dominant fifth relationship before falling back to a modal-mixture check against the parallel key. It applies a bounded rule order and states an ambiguous state honestly rather than forcing a label the evidence does not support.