I imagine there are as many answers as composers, but what I mean is…
Do composers write out the parts for all the instruments? I don’t know anything about music, but when I hear a symphony, obviously not all the instruments are playing the same tune…some do melody or harmony or rhythm or whatever.
Yes, a composer is responsible for every single note played by every single instrument.
Composers generally will write in the form of a score, which is the conductor’s part and has everything written down together. Then a copyist can simply extract the individual parts out of that.
I didn’t understand a word of that 
But it sounds like I’m getting closer to my answer! (I’m sorry, I know nothing about music.)
Can you explain more about that?
Thanks!!!
Jpeg Jones simply means that the composer will write every instrument’s part down on the same sheet of paper. But then, for the individual instrumentalists to play it, they’re going to need their specific part in front of them, not the whole entire thing.
Then the copyist (or assistant, or whoever is hired) will make sure that the violinists have the violin part on their music stand, the flutists have the flute part on theirs, and so on and so forth. The conductor is the only one who has the “full score”, or the printout of everybody’s parts on one sheet of paper, so he/she can follow along with everybody and cue each instrument on their entrances.
And you’re exactly right about not all instruments playing the same thing… sometimes the oboes, for example, will play the melody while the strings give them a nice padding of harmony underneath, or perhaps the French Horns will play the melody while harmonizing with a trombone and tuba. It’s all about mixing things up, and experimenting with different sounds (or “timbres”) that combining certain instruments create.
And let me add that it’s an incredibly pain-staking task. I took some composition courses in college, and occassionally we had to write short pieces for a full orchestra. Figuring out all of the instrument’s parts, how they’d sound together, what should play what when, etc., was very, very difficult – and this in the age of computers! How the great symphones of the 18th and 19th centuries were written by candlelight and by hand is simply beyond me.
And yeah, like DooWahDiddy said, the different instruments are typically playing different things, though having two different instruments playing the same thing together is not uncommon – just depends on the effect you want. Typically, one group of instruments will be playing the melody, while the rest provide the harmony and rhythm, etc. Listen carefully next time you hear a symphony and you should be able to hear this pretty clearly – the flutes will be doing one thing, the violin something else…
As a guy who has spent some time playing off either full scores or section (percussion, in this case) mini-scores, it’s not easy or fun. Percussion sheet music is often written with several parts on a staff (each getting a line) or sometimes the parts will be indicated and changing in the middle of the piece. There was one fairly short piece, can’t have been more than ten minutes and probably less, where the only percussion sheet music available was as a mini-score and was printed so that each part got its own staff (pretty much), making it so that only maybe 10 measures would fit on a page, creating an inordinate amount of page turns.
Mostly what has been said here. Traditionally there are forms that give a Symphony it’s shape, and sections that introduce themes or motifs. A Symphony at its heart is pretty much about theme, counter-theme and variation. Typically a composer will start with some motivic or thematic material (short snippets of melody or fractions thereof) and build from that. Themes are introduced by sections or combinations of instruments, then are deconstructed and extended.
A composer of a symphony will write out all the parts in a primary score paying attention to transposition for the instruments and then that score will (if the composer has the money) will be given to copyists to write out the parts. Modern composition programs shorten this process considerably and can extract parts from a complete score (so that the violin section has their own music, Bassoons their own copy of just their part and so on).
This is in contrast to most popular music and even Broadway scores which are typically written in a simple Piano-vocal score then given to Arrangers to prepare for a pit orchestra. Even Berstein used this device. He did his the arranging for his 3 symphonies and other serious work himself and often gave explicit instructions to arrangers for his Broadway scores.
So, in classical compositions the art is in the arranging, in popular music the arranger is in the art.
Hope this helps.
BTW: here’s what the Wikipedia has to say about symphonies: http://en.wikipedia.org/wiki/Symphony
To clarify/add to what’s been said before:
The string instruments are grouped into sections - ‘1st violins’ and ‘2nd violins’ have one part of music each, which all the members of the section plays, while the violas, cellos & basses each also have one part. Each string section can contain anywhere up to 20 or so players.
Wind and brass parts are written for individual instruments, so ‘1st horn’, ‘2nd clarinet’ etc. generally refers to an individual player. On occassions, these parts can be ‘doubled’, so two players play the same music, for extra volume.
You may also want to look at this short-ish Wikipedia article on orchestration, which is essentially the process of deciding what instruments play what parts.
Then how about a visual example?
This is the first page of the full score of the Brandenburg Concerto IV.
You can see running down the left side the parts for violin solo, flute I, flute II, violin I, and violin II. These are noted with a treble clef since the range of those instruments falls within the range of that denotation. The staves below do not repeat the “violin solo, flute I, flute II” bits — but that’s what they are. The cello and the double bass are written in the bass clef, and the viola is written in the alto clef. Since a harpsichord player has two hands, he gets two staves (one for each hand).
You can see that the violin soloist is basically playing with the first violin part for the first 12 bars. Then he has his own stuff to play.
This is the full score. It’s really a lot more than each individual player needs — that’s what the others are saying, that somebody will copy out a single part (like violin I) and give that to the first violins. Those players will just get a sheet of their own part without everybody else’s stuff.
Orchestrators do orchestrations for a living–deciding who plays what where. I image it’s quite a difficult job.
This brings up an interesting point: Bach didn’t quite write out all the parts for all the instruments note-for-note. Note that the harpsichord part says “editor’s realization.” That’s because in Bach’s time (early 18th century), it was common practice to write keyboard accompaniments in a kind of shorthand mix of notes and numbers called “figured bass.” The harpsichord player was expected to improvise his part based on these instructions.