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I am studying Music at AS-level - we have to harmonise a soprano line (in any style we wish I believe) with the alto, tenor and bass parts.

One possible perfect cadence I have been taught is the Ic-V-I progression. However I found the following graphic on a revision site. The two rules below it (about the bass being doubled, and the octave leap) are not things I have been taught. Are they true?

(It's possible that you would follow these rules if you wanted to be in a certain style, but doing so is not necessary for the exam I'm taking.)

Screenshot

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It seems like every theory program has a different name for this Ic construction. I admit I have never seen it called that, though I have seen a variety of other names, including I6/4, V6/4, and just plain not labeling the chord.

Nevertheless, the idea is the same. The chord labeled Ic is not a functional chord. Rather, it is a double suspension over the bass, and its resolution in common practice music needs to follow specific rules. Note that the intervals above the bass are (octave displacements of) a sixth and a fourth above the bass. The pitches happen to be the same pitches as the tonic (I) chord would have, but the chord cannot function as tonic. It must resolve to the dominant (V) chord with the suspended pitches going downward by step. That is, the sixth above the bass must go down to the fifth above the bass in the same voice, and the fourth above the bass must go down to the third above the bass in the same voice.

Because of the rules about downward resolution, the sixth and fourth above the bass may not be doubled in a four-part texture. To do otherwise would require parallel octaves in the resolution, which is (of course) forbidden in this style. Therefore, the bass must be doubled in the Ic sonority.

I have never heard nor seen the rule about the octave leap in the bass. If the Ic designation implies something beyond the designations with which I am more familiar, then perhaps it is a rule in that case, but I have seen many, many instances (at least in textbook examples) in which the bass simply stayed on the same pitch. I can imagine that such a rule would create difficulty in certain circumstances. Of course, the octave leap is permissible, provided that no other rules are broken.

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I agree- I think motion by unison and motion by octave are usually considered equivalent, although the octave motion is very characteristic. I was taught harmony with a strong suggestion to use the falling octave in this situation. –  cotroxell May 3 '11 at 22:03
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The Ic notation stems simply from the idea of appending a letter to count inversions. I(a) = major triad, Ib first inversion, Ic second inversion. –  ogerard May 8 '11 at 13:22
    
@ogerard Thanks. It's hard to believe I've never seen that before now, but I guess all my theory training has used figured bass (or figured-bass-like) symbols to indicate chord construction. –  Andrew May 9 '11 at 15:04
    
roman numerals vs figured bass seems the "great divide" in harmonic descriptions. –  James Tauber May 12 '11 at 8:54
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