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Context: My theory book is elaborating on some rules for melodic motion to be applied to the soprano voice when Writing in Four Parts.


In the screenshot below, I'm trying to digest the quote:

Two leaps of 4ths are usually poor, but arpeggiation of a dominant 7th can be effective.


Question: Are these two statements intended to be read in contrast to one another? I'm trying to imagine a scenario where a dominant 7th chord could be arpeggiated by two consecutive leaps of a 4th, but can't see it happening.

Maybe these are simply two independent rules which are joined with the word "but"? I'm just seeking clarification on which interpretation is correct.

Screenshot

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The two rules are meant to contrast each other. Two consecutive leaps of a (perfect) 4th combine to span a minor 7th--the same 7th interval as the one a dominant 7th chord uses.

An example of two consecutive leaps of a (perfect) 4th: F - Bb - Eb

An example of dominant 7th arpeggiation: F - A - C - Eb

Note how both examples begin and end with the same notes.

My experience is that two consecutive leaps of a perfect fourth sound dangerously close to a tuning exercise, and three such consecutive leaps sound even closer.

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Mathematically, there are only two ways to have two leaps in the same direction that span a 7th: two ascending fourths or a combination of a fifth and third.

In traditional part writing, two consecutive leaps of a fourth will sound out of place because they don't form a chord. Consecutive leaps of a third and a fifth (or a fifth and a third) can be effective in certain cases, most often outlining the dominant 7th chord. Remember that the dominant chord has a special place harmonically as the chord of highest tension, so it can be made to stand out more than other chords.

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