Quote:
Originally Posted by Guitars+gems
My son, who gave me the chord, said that too. B1/2dim7 I think he said. So, I know that a diminshed chord is one made up of 2 minor thirds. And I think a diminished 7th chord has 3 minor thirds? This Bmb7 is made of 2 minor thirds and a major third. I don't get where the "half-diminished" term comes from. I mean, if 2 of the 3 intervals are minor, why is it only half diminished?
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Your "stacking 3rds" concept is what's confusing you. Chords are best understood as intervals measured from the root (3rd-5th-7th etc):
"Major" chord = chord with major (larger) 3rd
"Minor" chord = chord with minor (smaller) 3rd
Both chords have a "perfect 5th" (7 semitones from the root), so it's the 3rds that distinguish them, give them their character, and is where they get their names. The interval between 3rd and 5th is irrelevant.
"Diminished" chord = chord with a diminished 5th (6 semitones from the root).
"Augmented" chord = chord with an augmented 5th (8 semitones from the root).
Here it gets a little more complicated, admittedly. The dim triad has a minor 3rd and the aug triad has a major 3rd, but in both cases it's the 5th that's more distinctive than the 3rd, giving each chord its essential dissonant character.
When it comes to 7th chords, the shorthand names get a little less consistent, but still refer to intervals from the root. Intervals between other chord tones still play no part in chord names.
"Major 7th" chord = chord with a major (larger) 7th interval. You can have "minor major 7th" chords too, which have a minor 3rd and major 7th. (See below)
"Diminished 7th" chord = chord with a diminished 7th interval. This is one semitone smaller than a minor 7th. The chord also has a diminished 5th (and minor 3rd), and is sometimes called a "full(y) diminished" chord.
"Half-diminished" chord = 7th chord with only one diminished interval - the 5th.
E.g.:
Bdim7 = B D F Ab (from C harmonic minor). B-F = diminished 5th, B-Ab = diminished 7th. Jazz symbol B°7.
Bm7b5 = B D F A. Just the one diminished interval, B-F. So only "half" diminished. (B-A is a "minor 7th" interval.) Jazz symbol Bø7, or just Bø. (The "7" is superfluous, because "ø" already says "half-diminished".)
BTW, when we call a chord a "minor 7th", we're actually referring to the 3rd of the chord, not the 7th - even though the 7th is minor too. This is how the shorthand works:
A7 = "A major minor 7th" in full. But chord symbol language takes a major 3rd and minor 7th as default, because those are the most common variants of those intervals. (In the major scale there are five minor 7th intervals and only two major 7ths.) So we can just call it "A seven", knowing the 3rd is major (C#) and the 7th is minor (G).*
Am7 = Here the "m" refers to the 3rd (C). The "7" is the standard minor (10-semitone) 7th (G). Jazz symbol A-7.
Amaj7 = The "maj" refers to the raised (larger) 7th (G#), not the 3rd (which is the default C#). Jazz symbol AΔ. (Think of the triangle as an arrow, raising the 7th.)
Am(maj7) = Now both 3rd and 7th have been changed from the default, so we have A C E G#. Jazz symbol AmΔ, or A-Δ
Like all good shorthand systems, the most common chord types get the shortest names!
* As you may know, this chord type is commonly called a "dominant 7th". That's because it's built on the dominant (V) degree of the scale (major or harmonic minor). I.e., A7 comes from the 5th (dominant) degree of the D major or D harmonic minor scale.
An A major triad is actually the "dominant" (V) chord in those keys. It becomes a "dominant 7th" when (surprise surprise) we add a 7th to it. The 7th (G, taken from the D major or D minor scale) is the smaller (minor) of the two common sizes; and the V degree of the scale is the only degree that produces that major 3rd-minor 7th combination, hence the "dominant 7th" name.