Music: Practice & Theory Stack Exchange is a question and answer site for musicians, students, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
In this concerto there are a couple of places where parallel octaves are used. I I am a harmony student but This is the first score that I have studied and see what just about every harmony book and counterpoint text teaches against. So what gives? Did Mozart not care about parallel perfect octaves and if they are so forbidden why do they appear in the very first score I have looked at? And most importantly, why are we taught not to use them?
Counterpoint theory generally handles the case of writing for a certain number of independent voices. Here it is a general rule to avoid certain parallels as these impair the independence of the voices.
An real world score such as this violin concerto is not made up of only independent voices. Take for example the violins in an orchestra: In a Mozart time orchestra you’d have about 6-8 first violins, all playing the same part. These can be seen as parallel unisons, but you would not consider them independent voices in the first place. And similarly not all parts of the score need to be independent voices in a contrapunctal sense. You will find though that on a large basis if you distill the core subjects and counterpoints from such a score there will not be such parallels, as these can quickly make the whole thing sound quirky.
The prohibition is when writing counterpoint, which Mozart is not doing. When one instrumental part doubles another, there is no attempt to maintain the independence of voices — in fact, specifically the opposite.
Counterpoint is one thing, this concerto is another. Voices here can (and do) get 'doubled', and as such, with so many different voices, those 'rules' have to be disobeyed.
As other answers indicate, it is because scores are often not engaged in "part writing" per se. The classical period is characterized by instrumental figurations that are not directly subject to voice leading rules. Such figurations were also common in the baroque period.
The middle voice of the violin triple stops might better be a D instead of an A at the downbeat of 497. (I'm no violinist, but I assume it's playable because the next chord has D in that position.) It's also possible that Mozart didn't write this piece (the attribution is uncertain) and that if he had he would have done it differently.
But in general, this passage is characterized by relatively free doubling. It's basically a three-voice texture from the voice leading perspective (as is often the case with a series of 6 chords). The violas are more or less doubling the basses, though not strictly so, just as the first violins are more or less doubling the first oboe. The part writing is basically this:
X:1
M:2/4
L:1/2
K:D
%%score {RH|LH}
V:RH clef=treble
[b d]|[a/ d][g/ d]|[f/ d]
V:LH clef=bass
G,|F,/B,/|A,/
The differing figures and the manner in which various parts jump from one voice to another are concerns of a different discipline from part writing: welcome to the world of orchestration.
To answer the question briefly and succinctly: because orchestral writing does not have the same principles nor is bound by the same rules as theoretical counterpoint. Or to quote the probably overused saying, this is apples and oranges.
When writing for an orchestra, there are various concerns (balance [between voices and between instrumental groups], texture, clarity) which do not exist when writing a mere counterpoint exercise. Doubling an important voice (in this instance: the bass, which is doubled [with octave jumps] by the violas; and the top note of the "melody", which is played by both the violins and the oboes) to give it more prominence is a natural solution to this.
In short, the issue here isn't one of counterpoint: these simply aren't independent voices (in the same way that the strings doubling the lower voices in this Bach cantata [BWV 182, mvt 7, see score image below] are not; or the flute and violin doubling the chorale melody at the octave later on are not; an independent voices), and thinking of it through the lens of counterpoint misses the forest for the trees. What you're looking at here is more properly called orchestration.
To answer your penultimate question (and with that put to rest any doubts about the final one), you will note that, if you disregard the doubling (in the Mozart as well as in the Bach example), the voice-leading still very much does follow the usual rules of voice-leading.
If one views music "theory" as making predictions about how various musical constructs will be perceived, what music theory says about parallel perfect intervals is that a group of instruments playing parallel octaves and fifths is apt to sound somewhat like a single instrument with a different tonal character.
Having some melody lines played by trumpets, some played by violins, and some by a tuba, is apt to make a piece more interesting than if every melody line were played by the same instrument, and having a melody line played mostly by violins except for a few carefully chosen accents which are played by trumpet can space things up, but only if transitions between instruments "make sense".
Likewise, using parallel fifths and parallel octaves is fine if the transitions between the portions of the music where lines move in that fashion and the portions where they move independently are sensibly placed in the context of the piece. Music theory suggests that such transitions may be perceived as being analogous to transitions between different instruments, but whether that's a good or a bad thing would depend upon what one is trying to accomplish.
