樂理問題 Q9 什麼是 PERFECT cadence?

其實多數情況都係要用root position o既....
不過如果真係唔係用root position 呢，都可以用3音(1st inversion)，例如C major， V chord GBD，都可以用B做底，但係就一定唔可以用5音， I chord 都係，多數用主音，都可以用3音，但一定唔可以用5音。

至於最高音(top note)，I chord 一定係要用主音，即係"do" 音，因為要咁先似完結。
V chord係自由選擇，但係一般都係會用3音同5音。例如你諗下，多數唱歌時，你會唱"ti--do"作結，咁" ti" 即係V chord入面o既3音，所以我地多數呢係用3音或者"ri---do" 五音作結。

希望幫到你﹗

唔使一定要root position
IV to I is plauge(可能串錯) cadence

Perfect cadence 是完美終止式！IV to I 是 Plagal 終止式，V to I 或 V7 to I 是 Perfect 終止式，I / II / IV / VI / Ic to V 是 Imperfect 終止式，V / V7 to VI 是Interrupted 終止式。

In fact, those V chords in the perfect cadence can be either in or not in root position. However, the perfect cadence with a V chord not in root position is usually called 'open cadence', and, the perfect cadence with the V chords in root position is called 'close cadence'.

P.S.Perfect cadence MUST BE V to I. Perfect authentic cadence, according to my knowledge, should be telling that the V chords in the cadence is in root position. And the other perfect cadence means that the V chords of the perfect cadence may not be in root position. But as you say, IV ro I is called plagal cadence. So, don't mix up.