The relation between voices in piano pieces

Question

I've been trying to understand how to identify the voices within the harmonic progression they form, after learning counterpoint and SATB harmony.

But when I'm investigating the relation between voices more closely (parallel octaves, fifths, harmony, etc) I see segments where the number of voices keep adding or changing. As a result, I can't really understand the connection between them. For example, I don't know where the alto is going because the number of voices has changed.

In the picture below, taken from one of Bach's sonatas, I suppose the voices are distinguished by their stem direction where the soprano is always upward and the bass downward, but am I correct that the number of inner voices may increase or decrease?

If I'm correct, here's a follow up question: does this mean we can have 2 altos when the number voices become 5, for example? I also see in Haydn pieces that the number of voices can sometimes go from 2 to 5; in that case, what became of the missing alto and tenor voices? Do they mix with each other?

If it goes down from 5 to 2 voices does that mean we now only have the bass and soprano voices? See measure 8 cadence resolution where you can see 5 voices. The bottom 2 D forming an octave in the left hand part; are they both just the bass, or is the higher D the tenor?

Answer 1

You should distinguish between contrapuntal style with true voices and actual vocal music and homophonic texture music or non-vocal music. The former will usually have a consistent number of voices, because each part is a distinct, independent line or an actual vocal part. The later is less about multiple, actual lines in counterpoint, and more about vertical chords.

Genres that you can associate with the two might be, for example, fugues, choral music, chamber music like string quartets for real distinct parts/voices.

Genres like solo keyboard music, continuo parts, symphonic writing, etc. can be homophonic (melody with bass and chord filler), where chords have a more vertical origin, rather than contrapuntal lines, and instruments do doubling and add coloring rather than act as independent parts/voices.

Answer 2

Keyboard pieces, even those in a contrapuntal style, often do not adhere to a strict number of voices. Voices may appear or disappear without warning. For example, the left hand of the Aria in Bach's Goldberg variations begins in three voices (to indicate that each note of the arpeggiated figure should be sustained) but has only two voices in the seventh and eighth measures. The same sort of thing is going on in this sonata.

It is not usually necessary to account for every voice at every point in time, and often it is unhelpful. When composers do want to be explicit about the disposition of the voices when there are fewer notes than voices, they will use multiple stems on a single note head to indicate doubling, or, as Bach does in the Goldberg aria, use rests to indicate that a voice is inactive.

Answer 3

The number of active voices can change within a piece, just like in a band not all instruments play all the time. It also may happen that a voice enters for a very short time. Think of that musician who sits quietly through the whole concert, just to crash cymbals once, at the end of the piece.

In the example you attached, there indeed seem to be 5 voices in the measure 7. I would say, the rhythm could have been notated a bit more clearly, but it seems obvious to me how it's intended to be played.

Answer 4

Just for kicks, I've scored the sonata for SATB, with alto and tenor divisi. Looking at it this was, I think, clarifies where the voices are. Although there are as many as six pitches being sung simultaneously, they still fit neatly into four "voices". This is an interpretation, of course, and others might assign parts differently.