Why are most string quartets in a key with few accidentals?

Question

The most accidentals I have seen in a string quartet is 4 flats or sharps. Now I have been told that this is because C major, G major, D major and A major are the easiest keys on a string instrument because they have the most open strings and the next easiest would be F major, Bb major, and Eb major. And of course the parallel minors of C, G, D, and A are relatively easy on a string instrument as well. I have also been told that unlike on a piano which uses equal temperament, string instruments use just intonation which means that a violinist has no desire to play C# major for example but for a pianist it is just as desirable as any other key is. I don't know why the tuning system would make certain keys not desirable. But the open strings makes sense, less fingerings so it is easier to play in C major which has all the open strings than F minor which has just C and G as open strings. F# major is actually the only key with no open strings at all. But then why is C minor relatively easy? It has only 2 open strings also. I guess it has to do with the parallel key of C major being easy. So, if I were to compose a string quartet which I plan to do at some point, no matter how much I like the key of C# major, I should probably avoid it at all costs and use C# major for a woodwind quartet or a piano piece instead, right and as a rule of thumb never go above 4 accidentals for a string quartet and instead try to reach the expressiveness of keys with more accidentals in other ways(such as a cello solo or lots of other ways). **But is it just the open strings as I speculate it is as to why I don't see string quartets composed in keys with more than 4 accidentals?**

Answer 1

Yes! You have some great points here, very well studied. Strings often sound best in keys which use their open strings allowing for a more vibrant sound (more overtones), and easier fingering throughout the instrument. This lends itself to sharp keys like you mentioned G, D, A, E. Flat keys often don’t ring as well on the instrument since there will be more be tension on the strings in these keys. I am definitely a fan of the key of C# or Db as well but it’s just not a great key for strings. I would also add that strings often sound much darker and lyrical than other instruments such as brass and woodwinds. Keys which are brighter such as D and A, actually might sound a bit darker with a string quartet. Strings often ring much richer in more spread voicings or even 2-part harmony in 3rd or 6ths (more overtones fill the empty spaces) compared to horns who played much closer and full voicings.