Would this cadence be considered perfect because of the root movement of dominant to tonic, or plagal because of the function and sound of the chords? Thinking here of like Eb/F, resolving to Bb.
Posted Feb 2, 2021, 3:55 PM UTC
Both, IMO. :-)
Technically, I suspect it has to be classed as perfect, because of the V-I root movement. But IMO an essential element of the V-I cadence is missing, namely the leading tone (A in this case). In place of that, you have a completely plagal (subdominant) harmony. So in that sense, it's much more "IV" than it is "V".
But (IMHO again), I think the single V root - because of its bass position - balances the 3-note IV harmony above; making it an equal mix of V and IV in effect.
I've seen this cadence called the "gospel cadence", which I think describes it nicely. It's certainly common in soul and gospel-influenced pop and jazz.
You can, of course, add the 5th of V without making a whole lot of difference. In this case a full F9sus4, or Cm7/F. The C then supports the F root a little more strongly (acoustically), but of course you now have a 4-note subdominant (or supertonic) harmony on top!
I'd also say both. If you classify it as an authentic cadence you could justify it with a 9-8 suspension if that's what the voice leading looks like.
I've always considered it to be both, Ie. the Eb resolving to Bb is plagal and the F bass note resolving to Bb is authentic.
Though I think if I had to label it as one or the other, I think it's always sounded a bit more plagal than authentic to me