It's actually a high A string for double bass, thinner than the common G string. Not only for solo tuning, but fifth tuning too. I prefer to use 5th tuning on my instrument, like CGDA. It extends the range up and down, and the double bass gets a new, brighter and stronger resonance too! The fingering won't be difficult, just try it. This string will help you all of the ways I wrote.
Lot of people worried about the fingering and the shifts in the 5th tuning system. I have got a 3/4 sized bass, and medium sized hands. But it's not a problem for me. Anyway, I tried to use this A string like a common G string, and it worked too. It wasn't so hard and tiring like a thick G string. I like it!
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