Open Strings of Huqin

Almost every huqin only 2 strings, one inner, one outer. And for Erhu(Terra), the inner string is D4 or d¹ and the outer string is A4 or a¹. For Gaohu(Terrin), the outer string is G4 or g¹ , the inner string is D5 or d². For Zhonghu(Terron), the open strings are one octave lower then Terrin’s, they are G3 or g(outer) and D4 or d¹(inner).

So, generally, the pairs of open strings of huqin are as follow:
Erhu       (Terra): DA - exactly the same D with Terron
Gaohu   (Terrin):GD
Zhonghu(Terron):GD - 1octave lower than Terrin.

Erhu(Terra)

 Gaohu(Terrin)

Zhonghu(Terron)