Checking out cadences for a student, I thought that 'perfect' was the U.K. version of the U.S. 'authentic'. Then went on to find 'perfect authentic' cadence. It seemed like tautology, but it appears to be to do with the voicing. Does this become an issue? 'Perfect' is V>I, or V>i: end of? How does the voicing change anything? What other different cadences are there that are more specific than the usual perfect/imperfect/interrupted/plagal?
Not only are there inconsistencies between UK and American terminology, there are inconsistencies within just the American terminology! You're 100% correct that an "authentic cadence" is
V–I, but it can be a bit more complex than that; here I'll give you the "academic" way of understanding classical cadences.
- At A, we see the common Perfect Authentic Cadence (PAC). It's labeled such because both the
Vchord and the
Ichord are in root position and the soprano ends on scale-degree 1.
- At B, we see the Imperfect Authentic Cadence (IAC). It's labeled this way because, although both the
Vchord and the
Ichord are in root position, the soprano ends not on scale-degree 1, but on scale-degree 3. (It could have also ended on scale-degree 5 and still been an IAC.)
From these two definitions, we see than an Authentic Cadence (AC) involves a
V chord moving to
I and both are in root position. This latter part can be contentious, but a number of music theorists, from Heinrich Schenker to William Caplin, have insisted on the
V chord also being in root position. (More on that later.) The distinction between Perfect and Imperfect solely rely on the soprano scale degree.
- At C, we see the standard Plagal Cadence. Note that some theorists don't consider this a "real" cadence per se because there's no
Vchord. I think that's silly, but from a standpoint of Schenkerian analysis, I understand why they say that. (In short, the Schenkerian mindset is that music is, at its most basic, a motion from tonic to dominant and back to tonic. If there's no dominant, there's no complete musical motion, thus no cadence.)
- At D, we have what we in America call the Deceptive Cadence (DC). It typically moves
V–vi, but really it can move from
anything that's not I. (Note that some distinguish a Perfect Deceptive Cadence from an Imperfect Deceptive Cadence based on what scale degree is in the soprano, but this is pretty rare.)
Now, here's where the inconsistencies between textbooks come in:
- At E, note that the cadence ends with
IV6–V6–I. Some textbooks (like the Kostka/Payne) will call this an IAC because the
Vis inverted. Though this is one way of viewing it, the majority of professional music theorists will call this a Contrapuntal Cadence (CC) on account of the stepwise motion in the bass. In short, if one or both of the final two chords is/are in inversion, it is a Contrapuntal Cadence.
- At F, we have a similar issue, but here both the final chords are in inversion: the
Vquickly moves through a
I6. You can call this a Contrapuntal Cadence, but it's such a common progression that it has earned its own name: an Evaded Cadence (due to how the bass quickly "evades" the feeling of closure with a root-position tonic chord).
Edit: Since I'm here I figured I might as well include some discussion of half cadences as well, even if they weren't addressed in the original question.
At G, we see the standard Half Cadence (HC). For a half cadence, all that matters is that it ends on a root-position
Vchord. (But again, some think a HC is possible with an inverted
Vchord.) There's also a disagreement on whether or not this
Vchord can include a chordal seventh. I think a
V7is fine to create a HC, but some disagree!
And at H, we see a more specific type of HC. Due to the half-step motion in the bass from the
V, this is called a Phrygian Half Cadence. This must be a half step in the bass; if we move from
Vin major, with a whole step in the bass, this just goes back to being a regular HC.
It does have to do with the voicing. I found a definition that explains it pretty clearly I think,
In a perfect authentic cadence, the dominant chord in root position is followed by that of the tonic in root position, and according to some theorists, the cadence is not "perfect" unless the uppermost voice is the tonic in the final chord.
An authentic cadence then I guess could be summed up as any old V - I.
I only remember from school this naming applying to this type of cadence, I do not remember anything like a perfect plagal cadence, or perfect authentic deceptive cadence.