IVm6 or IVm7? (in major)

Question

(Note: using the key of C major as generic instead of roman numerals.)

Yesterday I was reading the book "Hearin' the Changes" (by Coker, Knapp and Vincent), which discusses Fm7-B♭7-C as an ubiquitous formula called "backdoor cadence" used to return from tonal centre F to home key C.

However, the New Real Book's sheet for All of Me, for example, has F6-Fm6-C (measures 25-27).

Both formulas seem very similar in purpose (return from IV) and chords used. Both sources seem pretty reputable.

My question is: after an F chord (typically F triad or F6) established as tonal centre, which chord, Fm6 or Fm7, makes more sense to you? Why?

My own thoughts on this don't amount to much. I wonder if maybe Fm6 makes more sense when B♭7 is omitted and Fm7 makes more sense when B♭7 is present. However, I'm unsure about the use of Fm7 because generally I haven't seen the tone E♭ (or D♯) appear much at all (blues aside, such as in IV7 and ♯IVo7).

I'm particularly interested in the use of IVm in jazz standards, but any comments will be appreciated.

Answer 1

Both versions occur frequently. As usually, let your ears decide. Theoretically speaking, the Fm6 chord just uses one note outside of the C major scale (the third A♭), whereas Fm7 also uses an E♭. So by using Fm7 you temporarily move a bit further outside the scale. It's true that if the Fm chord is followed by a B♭7 chord, then it's more natural to use Fm7, simply because then you have a standard II-V unit, and you have the additional resolution E♭ → D when moving from Fm7 to B♭7.

Note that there's also a third option, which occurs less often, but which is also interesting: Fm maj7: F - A♭ - C - E. Just like Fm6 it only lowers the A to A♭ without changing any other scale notes.

Also note that the corresponding chord scales are different. For Fm6 and Fm maj7 you would normally use the F melodic minor scale, whereas for Fm7 (or Fm7-B♭7) you would use F dorian ( = B♭ mixolydian = C aeolian).

Answer 2

As the other answers suggest, both options have equal validity when considered exclusively from the perspective of a successful cadence, because they both provide good resolution back to the I chord. So the question "which makes more sense?" won't have an answer--they are equally correct, and both are used.

However, if we leave the vacuum of pure cadential expectations, then (a) melody, (b) soloist preference, and (c) musical genre/style can dictate why one progression might be preferred over another.

Let's consider measure 12 of "When the Saints Go Marching In," assuming we're working in CMaj. As MattL explains, there are three options:

• | Fm6 | CΔ7 |
• | Fm7 Bb7 | CΔ7 |
• | FmΔ7 | CΔ7 | (the simplest spelling of FmΔ7 is F-Ab-C-E, called "F minor major seven")

Which of these do we use in m. 12? We would probably choose Fm6 or FmΔ7, because the melody features an E♮ in m. 12. So Fm7 could clash with the melody because Fm7 contains an Eb rather than E♮.

In other circumstances, the soloist or person improvising might express a preference. If a soloist starts playing an Eb over measure 12 of "When The Saints Go Marching In," then the band would probably want to adjust the Fm6 to Fm7 or to Fm7-Bb7.

Finally, when composing, you can evaluate the style you are working within. ii-V progressions tend to be more common in swing and bebop, whereas Fm6-CΔ7 might tend to be more typical of gospel, rock, blues, bluegrass, or popular music. These are not black-and-white rules, and you can certainly make either progression work in any genre, but both options are not used with equal frequency in all genres.

Answer 3

I won't write a long answer. The theory explained in other answers is good. Whether you want to play Fm6 or Fm7 largely depends on the kind of music you desire to play.

Answer 4

C7-F6-Fm6-C is called the sub-dominant cadence in Jazz (o when the saints go marchin' in), also as final ending below the octave note C.

Fm7-B♭-C is equivalent with Ab6-B♭-C (as you say the "back door cadence). Billy Shears ending cadence.

Mostly they are exchangeable - if the main-tune allows it! If you play alone or arrange a piece you can do what you like. If you play or sing with others, listen carefully what they're doing!