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I am nearing the end of composing the first movement of my suite. And I plan to end the movement using a plagal cadence. But I know that a plagal cadence can be hard to make sound convincing. I mean after all, it is a subdominant to tonic motion. And most of the time if anything, the subdominant moves to the dominant for your classic IV V I or a slightly embellished version of the same cadence such as IV V vii°7 I

I know that part of making a plagal cadence sound convincing is to slow it down. V I could easily be convincing at quarter note speed. Not so easy for IV I to be convincing at that speed. But as you slow it down, IV I becomes more convincing as an ending cadence. Another thing that makes the plagal cadence more convincing is having the upper voice move between scale degrees 5 and 6, implying a second inversion subdominant chord.

Second inversion is the least stable form for any major or minor triad. And scale degrees 6 and 4 naturally want to move downwards. This leads to a convincing tonic resolution from the subdominant. Repeating the chords in a pattern like this:

I IV IV I

as it is in the Messiah Chorus, makes the cadence even more convincing.

But do I need to do anything else to make the plagal cadence convincing? Would a leaping bass make it more convincing like it does for an authentic cadence?

Often the plagal "cadence" is best understood as post-cadential. In other words, the plagal motion really functions to confirm a tonic already clarified by a prior cadence.

In fact, this is what Handel does in the "Hallelujah" chorus. The perfect authentic cadence appears at the end of "and He shall reign forever and ever." The plagal motion (all of the subsequent "forever, and ever, Hallelujah"s) then comes after.

As such, the best way to make a plagal cadence sound convincing, perhaps paradoxically, is to make the preceding perfect authentic cadence convincing.

Don't worry about it. Hymns end with plagal cadences all the time, and we always know when they've finished perfect satisfactorily!

Yes, a strong bass line always helps an ending. Use root position IV - I chords, or root both chords on the tonic.

You can have the best of both worlds by rooting IV with the dominant note. Still technically a plagal cadence? Who cares?

But you're approaching this in a strange way. End with a plagal cadence if it's right for this particular piece of music. Why force one in? Most composers, most of the time, have decided it isn't and written perfect cadences. Perhaps they're right?

Try emphasizing the movement between the 4th scale degree (root of the IV chord) to the 3rd scale degree (3rd of the I chord). The half step resolution down acts similarly to the leading tone in the V chord to the root of the I chord. A plagal cadence will never sound as final as a perfect authentic cadence, however, since that's just not the nature of it. Our ears have cued into that V - I sound as being an indication of finality, and a IV - I will always sound a little more mellow in its finish.

As a composer, I don't worry about the standard cadences. I have pieces that use IV6 in a "half cadence" setting. I am working on one song that ends iv-i in a natural minor setting where I don't use a leading tone once. I have another song ending VII-i in minor. In some cases, I find the traditional authentic cadence too harsh. What is important is whether the music is communicating the mood desired.

In terms of making harmonic movement stronger, including at cadence points, moving the bass line in 4ths and 5ths is very effective. When using chords in inversion, the ear can be "tricked" into hearing the harmony according to the bass line, and the higher voices become more of a color, especially when using extended chords with 7ths and beyond.

The melody is also extremely important because this is where certain notes can be emphasized as landing points. If you shape your melody properly, the line can lead into the resting note, both through the use of tonal direction and rhythm. (I end another song on the 3rd of the key rather than the tonic.) Pay special attention to the use of semi-tones in directing the melody.

More than anything, good voice leading is important. You can make a piece go anywhere you want, but the smoother the transitions the more natural and convincing it is. A very angular approach might be interesting, but might also make one wonder if the music was supposed to go that way. On the other hand, a smooth transition to an unusual harmonic place can communicate that the music was obviously supposed to go there.

In short, if your piece feels like it ends at the end, it is convincing. However, even that is not necesssary. If one is writing in mixolydian, for example, the final cadence will always sound like a half-cadence and why shouldn't it? It is only common usage that has made the ear require or expect a more final sound. The level of drama and intensity of an ending will be affected by the type of cadence used, but what is desired should be determined by the mood, style, and mode of the piece rather than what is standard.

I'm always happy to hear someone say that there are no hard fast rules in music theory, because I've noticed in my own composing and separately the performance of those same compositions, I tend to apply cadences according to how I'm feeling at any given time. When I compose I may write in a V-I cadence but in performance hear IV-I in my head and without thinking play that instead. Needless to say I'm not sight reading at the time and I do a lot of solo performing so I'm not to concerned with throwing others off at the time, but it can change how I use cadences according to my mood and situation, and I'm glad I have that freedom.

The plagal cadence is like the amen in the church a traditional and commun used progression that we hear it as a final clausle even stronger as a perfect cadence. It sounds most convincing - not because of some theoretical reasons - but because of our usual habits. There is no law or rule that can explain this result better than the assoziation we have with existing songs and choral or orchestral operas. The effect is the same as the minor iv or the iib5 in the 1st inversion. The subdominant chord can be notated in the root position, but it can also stay on the root note of the dominant. There are lot of songs also ending on the 3rd inversion of the iib56, that means the soprano plays (sings) do-re-mi and the bass do-lu-do (lu = the minor third of iv).

All this closes will be convincing as ending by the use of commun practice.