Let's say I have a two chord progression written in four-part harmony that consists of a I chord and a V6 chord in the scale of C major in 4/4 time. The bottom 3 notes (Alto, Tenor, Bass) of both chords are whole notes. The top-line melody notes (soprano) are quarter notes (last top-line melody note is a whole note). Let's say I compose a melodic line that consists of the notes C, D, E, F, G. The note C starts on the I chord on the down beat of measure 1 and the note G ends on the V6 chord on the down beat of measure 2. Using this example, when checking for parallels between strong-beats, do I have to check for parallels between beat 1 and beat 3 in the first measure, and do I have to check for parallels between beat 3 in the first measure and beat 1 in the second measure? When checking for parallels between weak-beats and strong-beats, do I have to check for parallels between beat 1 and beat 2, beat 2 and beat 3, and beat 3 and beat 4 in the first measure, and do I have to check for parallels between beat 4 in the first measure and beat 1 in the second measure?
Since the lower three voices don't move in the first measure, there can't be parallels within that measure.
For certain you have to check m.1 b.4 moving to m.2 b.1.
That leaves whether you have to check for any other parallels between m.1 b.1–3 and m.2 b.1.
- Strong beats must be checked, which means, for certain, m.1 b.1 and m.2 b.1.
- In Fux's rules of counterpoint, only beat 1 is a strong beat, so beats 2 and 3 are okay.
Recognizing the example given is intended to illustrate a general issue, it's still instructive to consider it specifically. Assuming good voice-leading in the lower voices, all chord tones either descend by step or remain the same between m.1 and m.2: C -> B; E -> D; G -> G. Since the soprano voice is ascending, there cannot be parallel or hidden fifths or octaves.