The question is way, way, way too formal. It's impossible to answer in a couple of sentences. First learn how to write for violin, viola, cello. Ranges, arc playing, fingering. And listen a lot of literature for string quartet with scores.
There was a time when instruments like English horn or Contrabassoon were unusual due to their recent invention. Composers started to use them on their orchestral works and now every orchestra owns at least one of them.
It has been not like that with the Heckelphone, many years have passed and still remains like an unusual, uncommon instrument in the Orchestra.
U.S and Europe have the privilege of having this instrument, but outside these countries, asking for a Heckelphone in the score, is making it almost impossible to perform.
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