Which two voices below make up an interval of an octave?

Question

BlackTom AI · Accepted Answer

An octave is the interval between two notes that share the same letter name and pitch class but are eight diatonic scale degrees (or 12 semitones) apart.

1) Relevant concepts
- In four-part (SATB) notation the treble staff usually contains soprano (top) and alto (lower) voices; the bass staff contains tenor (upper) and bass (lower) voices.  
- Octave counting is inclusive: same note name counted from 1 to 8 (C–C = 8). An octave = 12 semitones.

2) Step‑by‑step identification
- Locate the two notes shown on the grand staff and identify which voice each belongs to: the lower note on the treble staff is the alto, and the lower note on the bass staff is the bass voice.  
- Observe the accidentals: both notes have matching sharps, so they are the same pitch class (for example, both could be F#), only in different octaves.

3) Interval measurement (calculation)
- Count diatonic steps inclusively from the lower pitch to the higher pitch: bass (1) → ... → alto (8) = an octave.  
- In semitones: same letter name one octave apart = 12 semitones (e.g., F#3 → F#4 = 12 semitones).

4) Why the correct answer is correct
- The pictured lower staff note (bass) and the lower treble staff note (alto) are identical pitch classes separated by exactly one octave; that is the defining property of an octave.

5) Why the other options are incorrect
- Tenor and soprano: those are the upper notes of the two staves and in the picture they are not the same letter name an octave apart.  
- Bass and soprano: these are farther apart than an octave (they span more than eight diatonic degrees in the engraving).  
- Tenor and alto: these are close in range but not the same pitch class separated by an octave.

Conclusion: The interval shown is an octave formed by the bass and alto voices. This confirms the correct answer: bass and alto.

Which two voices below make up an interval of an octave? 单项选择题

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