How to work with the chord builder
The module names a chord symbol and has you select the notes it contains, or it gives notes and asks for the symbol. The first levels cover major and minor triads, then diminished and augmented, and finally seventh chords and inversions.
Think in stacked thirds for every question, not in memorised shapes. Root, a major or minor third above it, then the next third. Follow that path consistently and you can derive any chord, including ones you have never seen.
Say the notes out loud while selecting them. It sounds silly and it works: chord building is arithmetic, and speaking aloud stops you from skipping a step.
From triad to chord symbol
A chord symbol is shorthand. The letter names the root; everything else describes what is built on it. A bare C means the major triad C–E–G. A small m replaces the major third with a minor one, so Cm is C–E♭–G. An added 7 puts a minor seventh on top, maj7 a major seventh.
Extensions follow the same logic. Sus4 replaces the third with the fourth, sus2 with the second; neither chord has a major or minor quality, which is why both sound suspended. A 6 adds the sixth, a 9 the ninth, and in practice the seventh usually sounds too. A slash symbol such as C/G means the chord sits over a different bass note, here the second inversion.
Inversions change the impression, not the chord. C–E–G, E–G–C and G–C–E contain the same notes and are all called C major. Which inversion makes sense is decided by voice leading: usually the one where the notes move least on the way to the next chord.
From practice
The diatonic chords of a key are the fastest route to usable knowledge. Build a triad on every degree of a major scale and the same pattern always appears: major, minor, minor, major, major, minor, diminished. In C major that is C, Dm, Em, F, G, Am, B diminished. With that single pattern you can predict the chords of almost any song in a major key.
It makes sense to analyse the chords of a song you know rather than practising new chords in isolation. Almost always it turns out that the song uses only the diatonic chords, and that discovery produces more understanding than any table.
For guitarists it is worth looking at what a shape actually contains. An open G major shape doubles notes and sometimes omits the fifth; that is normal and shows that a chord is a set of notes rather than a fixed arrangement.
Common mistakes
The most frequent mistake is confusing m7 and maj7. The small m refers to the third, the maj to the seventh. Cm7 has a minor third and a minor seventh, Cmaj7 a major third and a major seventh, C7 a major third and a minor seventh. Writing those three side by side settles it in a minute.
The second mistake is forgetting accidentals in remote keys. F♯ major consists of F♯, A♯ and C♯, not F♯, B♭ and D♭. The sound would be the same and the spelling wrong.
The third mistake is treating inversions as different chords. Looking for a chord rooted on E when you see E–G–C leads nowhere. The question is always: which notes are present, and which of them is the root of the stack of thirds?