Clarinet is my main instrument, but I also play the alto saxophone. I, a woodwind player, am attempting to compose a string quintet. My question is this: what are the physical limitations of the violin, viola, cello, and double bass? What is the maximum amount of notes that can be played at once? How do I avoid two notes that would have to be played on the same string (which is impossible)? Can a string player sustain a long note while playing a moving part on a different string? A side question: is it common for string instruments to play triads, or are they usually split up between sections? If the answer is yes, what instruments is it common on? I'm sorry if this is a lot of questions, but I'm still pretty new to composing for large ensembles. I appreciate any help. Thanks in advance!
What is the maximum amount of notes that can be played at once?
Two, in most cases. Three can be done if you loosen the bow hair significantly, but I wouldn't recommend writing it.
How do I avoid two notes that would have to be played on the same string (which is impossible)?
The only notes that can only be played on one string are the lowest ones on the lowest string, below the second open string. For example, low G to C# on a violin. So a low G-B double stop would be impossible.
Can a string player sustain a long note while playing a moving part on a different string?
This is common and not too difficult in fiddle music, where the sustained note is an open string. Doing it with a fingered note gets difficult quickly. You can write this, but take very careful care about how it would have to be fingered.
A side question: is it common for string instruments to play triads, or are they usually split up between sections?
Sustaining three or more notes requires dividing. It's not uncommon for solo literature to include 3- or 4-note chords, but they have to be arpeggiated and only the top 1 or 2 notes are sustained.
I agree with everything Mattaputnam has said. However, the best thing you can do to learn how to write a string quartet is to read some string quartet scores. There are tons of them out the there; the Beethoven string quartets are some of my favorites.
There are many techniques that are technically possible but very awkward to do. You will get better sound and happier musicians if you stick with what's comfortable to play until you get to know the instruments better.