There have been other threads on this same subject in which the discussion has gone off in all kinds of confused directions, partly caused by several people quoting the definition for "chord" in the Grove Dictionary. For all its reputation and prestige, the Grove Dictionary's definition is incomplete and misleading. What I've written below comes from two years at the University of Washington School of Music, two years at the Cornish School of the Arts, several books on music theory by such people as Walter Piston, Paul Hindemuth, and others, and many private lessons in music theory and composition from Mildred Hunt Harris. So this is the straight scoop.

The basic answer is simple, but it is subject to all kinds of permutations: It takes three notes—a triad—to constitute a chord. By definition.

A chord is not just any indiscriminate pile of notes. A chord must be at least a triad. A triad consists of a root (the name note of the chord), a 3rd (a major or minor 3rd above the root), and a 5th (a perfect 5th above the root). Example: a C major chord is made up of C, E, and G. A C minor chord is made up of C, Eb, and G.

You can double notes in other octaves (C, E, G, C, and E, such as the first position C major chord on the top five strings of the guitar) and it stays a C major chord. If you leave notes out, it's no longer a triad, hence, no longer a chord (although most people will refer to it as a chord, even though it's not complete). It needs the 3rd (E or Eb) to identify it as either major or minor. If you leave the E or Eb out and play just Cs and Gs, you have what some people call a "power chord." But it is an interval of a perfect 5th, not a complete chord. If you play a C and an E, but leave the G out, you imply a C major chord, but it isn't a complete chord.

You can add notes to a basic triad. Add an A to a C major and you have a C6. Add a Bb and it's a C7. Add a D and it's a C9. And so on.

You can also invert chords. A C major chord with the E in the bass instead of the C is in first inversion. G in the bass makes it second inversion. There's no third inversion because that puts it back in root position again — unless it's an added note chord such as a C6 with and A in the bass.

But if you're dealing with added note chords, you have to be especially aware if you begin leaving other notes out. For example, if you add an A to a C major, going for a C6, but you leave the G out (say, C, E, A, C, E on the top five strings of the guitar), you no longer have a C chord, you have an A minor chord in first inversion (C, E, and A, with the C and the E doubled).

Probably more than you ever wanted to know about a C chord. Of course the same stuff (basic triad, leaving notes out, adding notes, and inversions) applies to every chord. It's all very orderly.

This is part of the "mathematics" of chords. But nobody's going to shoot you if you play a first inversion Am instead of a C6 as long as it sounds good in context. That's the ultimate criterion.

That's about it.

Don Firth