Many of the songs here on the site include obbligatos that specify the use of a “C instrument.” People often write to ask, “What exactly is a C instrument?”
So without further ado…
A “C instrument” is any instrument that plays a real, honest-to-goodness C when it encounters a C on the staff.
Sometimes the instrument will sound in an octave other than the one you expect, but in spite of this liberty, it’s still a C instrument. Even violas, who insist on a clef of their very own, are C instruments.
If that list is too long, the C instruments I most often write for are Violin, Cello, Flute, and Oboe, with some chimes thrown in here and there. They are sometimes interchangeable, sometimes interchangeable with a few adjustments, and sometimes interchangeable if you don’t mind annoying the player mightily and/or providing chocolate.
If an instrument isn’t a C instrument, it’s a transposing instrument. These renegades see what cellos and flutes and oboes render as real Cs, and make some other sound. When you listen to an orchestra, a substantial number of the instruments are doing this. Actually, they do it for every note, C or not. The grand thing is that somehow, like magic, it works.
I don’t often specify obbligatos for transposing instruments, although some of the obbligatos would sound just fine played on a horn or clarinet or alto flute or… If you have such instruments at your disposal, you can transpose the obbligatos using the Personal Composer files/demo. Some players are skilled enough to read the obbligatos as written and transpose on the fly, but don’t push your luck.
A related question that comes up is, “Will this obbligato sound good on a…” Unless the sentence ends in “kazoo” or “washboard,” give it a try and see if you like it. If the obbligato specifies “C instrument,” it’s probably simple enough to sound reasonable on any of the more commonly-available instruments. If it specifies a “Flute” or “Violin” (or whatever) chances are I was after a particular sound or wrote in a particular register–-though that doesn’t mean it won’t work on other instruments. The bottom line here is… if you like it, use it!