I am learning all about subdominant triads and my textbook shows most voice leading of I-ii-V progressions using major keys. It says that because of its harsh quality, the ii° chord was not used as much and that the ii°6 would have been more common. If so, wasn't there any subdominant harmony built on the 2nd degree in minor? Since ii°6 and iv both are built on 4^.
I would say yes, but with some caveats.
If we are talking about the proper Classical era, there were a lot less minor key compositions, so that will make minor
iio less frequent.
Overall, the commonest chords by far are
IV V I, everything else is much less common if you count all chord occurrences in a composition.
ii becomes a contender along side
IV as the common subdominant chord.
...because of its harsh quality, the ii° chord was not used as much...
Maybe there is more discussion in the book, but as given, I'm skeptical. Common practice harmony is primarily about tonic/dominant and the other chord support that. The less frequent occurrence of the other chord should not be misconstrued as avoiding some problem. Consider and even less common chord, the augmented sixth chords. The is no question that chord or the
iio are part of the common practice harmonic vocabulary. You might argue the focus on tonic/dominant was about some kind of dissonance avoidance, but the theory I have read says it was about rhythmic style and the structure of sonata form. But let's not get into all that.
One final thought about questions of chord frequency and style. In the chorale style music that many of your recent questions deal with the harmony is much denser than other common practice forms. Historically it reaches back before the Classical era. In chorales it's easier to find minor key works, phrases are short, modulations/tonicizations frequent, and the modal regions are more common. In other words chorales provides lots of harmonic variety, especially Bach's Harmonized Chorales. This can skew the sense of what is "common." But, it also means chorales are a good place to look for examples of chords in use. It's also why chorale harmonization is a common harmony exercise.
You might like these articles from David Temperley. A Statistical Analysis of Tonal Harmony is directly relevant to your question. But also look at the articles with "rock" in the title. There are two statistical analyses of progressions in rock music.
...wasn't there any subdominant harmony built on the 2nd degree in minor?
I completely overlooked this part of the question when I first answered.
Common practice style is the context so I think the easiest way to look at this is through figured bass harmony which was the way harmony was taught at that time.
A general figured bass concept is to play root position chords on bass tones of the tonic or dominant and play inverted "chords of the sixth" on all other degrees. That makes
V the stable chords and sets up the emphasis on tonic/dominant harmony in the common practice. You wouldn't harmonize a bass of scale degree
^2 with a root position chord, because that treatment is for stable points of tonic/dominant.
^2 would be harmonized as a
6/4/3 chord which means it's normally
V4/3. This basic figured bass treatment was the same in both major and minor keys.
Maybe it could be argued that
iio6 is less dissonant that root position
iio, perhaps because there isn't a tritone in relation to the bass, and so was less commonly used, but it seem much more straight forward to explain
iio6 was more commonly used, because of the conventions of figured bass harmony.
From what I remember (from reading various harmony books and some history of music), the ii0-V-i was very popular from about 1600 on. There are a couple of caveats as mentioned by Michael; one is that the ii0 of occurs as ii06 (in first inversion) where it's not so harsh. Second, in sequences (like repetition) can generate forgiveness for many sins (in music, not law or religion); if the ii0 occurs in a cycle of fifths context i-iv-VII-III-VI-ii0-V-i, the succession of fifths (or fourths) in the bass makes the ii0 in root position sound like it belongs. In addition, sevenths are often added to the ii0 and V to give a bit more color. One often sees v065-V7-i (or been v00-i64-V7-i). All of these are an extended authentic cadence (not necessarily perfect). I'v seen other slight changes (to emphasize the ending) such as i-iv-VII-III-VI-ii0-v-i-i-vi-VII-III-VI-ii065-V7-i (or even I).
As an aside, I have used a minor Romanesca variation (related to Pachelbel's) as i-v6-VII-III-iv-I65-ii07-v-i-v6-VII-III-iv-I6-ii065-V7-i for the same reason.
No: the chord sequence in your notation doesn't occur, because it implies chords which would be analysed as something simpler instead.
In this answer I shall use
^2 etc. to denote the degrees of the scale relative to the tonic. In actual print these may be rendered as digits with the
^ on top of them.
You use a lower case i, indicating a minor key, so I'll take A minor as my key for examples.
The ° notation denotes the diminished triad. This is formed by stacking two minor thirds on its root. ii° is such a chord whose root is
^2. In A minor,
^2 is B so ii° consists of the pitch classes B, D and F.
V is a major triad on
^5. In A minor,
^5 is E, so this chord consists of E, G♯ and B. The chord has no G, but has G#.
But I would not call your progression ii° V i. The reason is that there is a simpler harmonic explanation for these chords: it is simply V7 i, where a dissonant note is struck at the same time as the V chord is struck, and then resolves while the V chord is still sounding. In A minor, the chord V7 has E, G♯, B and D; simultaneously with that chord, we have the dissonant F, which resolves onto E.
To take an even more reductionist view, we have V i. In A minor, the chord V has E, G♯ and B; simultaneously with that chord, we have the dissonant D and F, which resolve onto E.
On another notational point. If the notation for a chord includes a number as an ordinary numeral (not a Roman numeral), it denotes a pitch specified relative to the root of the chord. (There's a further issue in that Roman numeral notation e.g. vi7, V7 has one convention, and jazz chord notation e.g. Am7, G7 has a different convention, regarding the sharps and flats, but that is not relevant to the current point so I won't go into it further.)
The chord notation up to and including that number (e.g. V7) specifies the chord's pitches (relative to the tonic). But it does not specify the chord layout --- it does not specify in which octave(s) each pitch occurs. And in particular it does not specify which pitch is the lowest one. If the lowest pitch is not the chord's root, the chord is inverted. If the lowest pitch is the chord's third, fifth or seventh, the chord is in first inversion, second inversion or third inversion respectively. These are indicated by the letter b, c or d respectively. at the end of the chord notation.
For example, in A minor, V7 consists of the pitches E, G♯, B and D. If the lowest pitch is a G♯, we'd write V7b. Not V6 or anything 6.
II- V - I is just a very good chord progression if you think about it critically. When you start your journey with 4-part harmony the first seventh chord you learn about is the one built on the dominant note of the scale. The second one you learn about is the one built on the super tonic.
The 2, 5, 1 progression is soo good because the seventh of the super tonic chord is the tonic note of the scale. Now this resolves down to the leading-tone of the scale (like all chordal seventh do), which is in turn the third of the 5 (dominant) chord, which in turn resolves back up to the root note of the tonic chord. All excellent in teaching the concepts of good voice leading to the students.