What's the difference between a Gm7b5 and an E♭9#11? It seems there's no difference. Can someone with sufficient knowledge please help out with this. I'm trying to learn this song.
A Gm7b5 is a half-diminished chord. A Eb9#11 is a dominant [11th] chord. The main difference between the two, other than the notes, is the way they resolve.
A dominant chord almost always resolves to the tonic of the key, so in this case the Eb9#11 should resolve to an Ab major/minor chord (it should probably be an Ab add9 (Ab, C, Eb, Bb) if the #11 is properly being resolved). Of course there are deceptive resolutions such as resolving to vi, bVI (in major keys), VI (in minor keys), and other stuff such as dropping the bass note to the 7th of the chord (4/2 inversion), which lead to the first inversion of the tonic (i6/I6). In Chopin's Nocturne Op.48, No.1 in C minor he tricks the listener 3 bars before the coda: we expect to hear the tonic chord after the G7 which would end the firey
doppio morimento section. Instead we hear a V4/2 of bII (Gb, Ab, C, Eb), which is highly jarring upon the first listening as you expect to hear a simple return to the tonic and then a coda. It then leads to a bII6 (F, Ab, Db), which subsequently leads to the coda with a simple V7-i motion. Chopin never fails to deliver!
Half-diminished chords are fun little chords, as they can be used in a variety of ways. They mainly play a pre-dominant function in traditional 4-voice harmony, meaning that the next chord following them will usually be the dominant chord of the given key. They do NOT function as dominant role, even if they contain certain notes of the dominant chord in the given key. Take the two chords we have in the MJ song for example, the Gm7b5 consists of the notes G, Bb, Db, F; the Eb9#11 consists of Eb, G, Bb, Db, F, A (natural). You can see that the half-diminished chord consists of the third, 5th, 7th and 9th of the dominant chord. Why then is it not considered a dominant function chord? The answer is simple: it doesn't provide a strong cadential motion. The strongest cadence is a V-I motion, where scale degree 5 in the bass resolves to scale degree 1. This is why the Eb9#11 going to Ab would be considered dominant, because it's bass is scale degree 5 in the given key (Ab) going to scale degree 1. The bass note of Gm7b5 is scale degree 7, which is NOT a strong motion when going to the tonic- it isn't considered to be a cadence at all actually. My theory professor use to rant on about how half diminished chords are just half-assed dominant chords, which is true in a sense. They do provide a mysterious colour to music, especially in Impressionist and Jazz music I find.
Sorry for ranting on for so long, but there is a LOT that can be discussed between these two chords and their roles. Tell me if you want me to clear up anything I said here.
Topic: Trouble with 9 11 chords