Theory Level: Intermediate
It’s Bb Augmented. That’s the answer to last week’s question. The only triad that is properly spelled with both a sharp and a flat. In this case Bb, D, F#.
But why is such a seemingly normal chord the one exception? Well, right off the bat, if you know a little theory, you probably should have guessed it would be an augmented chord. After all, every major, minor or diminished chord is diatonic in at least one major key, and since no major key signature contains both sharps and flats, it stands to reason that no major, minor, or diminished chord could possibly satisfy the requirement. And that only leaves augmented chords as possible answers. But I’ve already lost half of you, so let’s back up.
The building block chords of tertian harmony (the most common system of western harmony) are triads, made by stacking two thirds on top of one another, that is starting on a root note, going up two letters, then going up two more letters (e.g. C, E, G, which is C Major). However, knowing that we go up two scale degrees (letters) doesn’t tell us everything we need to know about a chord, because notes can be sharp, natural, or flat. And thirds can be either 3 half steps (a minor third) or 4 half steps (a major third). Half steps are the smallest interval in standard western tuning, equal to a single piano key (white or black) or a single fret on the guitar. The closest two notes can be to one another in standard tuning is one half step, and a third can contain EITHER 3 OR 4 half steps. So if I go up a minor third (3 half steps) from A I’ll get C but if I go up a major third (4 half steps) I’ll get C#. Both are thirds and both are 2 letters apart. This is an important concept for later.
So, if we go up a third and then another third, there are 4 possible combinations of distances between our 3 notes. 3+3 (which we call a diminished chord), 3+4 (minor), 4+3 (major), or 4+4 (augmented). Using the example root C we can get C diminished (C, Eb, Gb), C minor (C, Eb, G), C major (C, E, G), or C augmented (C, E, G#). But notice that no matter which triad we build off the root C we will end up with the same 3 letters: C, E, and G. That means that since there are only 7 letters in our musical alphabet there are only 7 combinations of letters that can form triads, and memorizing them is pretty easy. This makes tertian harmony very intuitive if you understand the basic theory, and is really quite a brilliant way to organize pitches and the chords formed from them.
It also means that even though two notes might be enharmonic to one another (the same piano key), for instance C# and Db, the two are not interchangeable when spelling chords. Hence, in our Bb augmented, we cannot spell the chord Bb, D, Gb, because even though F# and Gb are the same key on the keyboard, from a spelling standpoint, Gb is three letters above D, not 2 like F#. No G note can ever be part of a triad with Bb as the root. So F# is the only correct spelling. But that still doesn’t tell us why this one chord is so special.
Next week…