When it's Bluegrass it's Fiddle Racerback Crop Top - Women's 

The Bluescentric Brand official Bluegrass Racerback Crop Top comes in relaxed cropped-fit sizes Small through 2XL. It is 52% airlume combed/ring-spun cotton, 48% poly jersey, racerback and features raw edge (unsewn) arm holes.

It's not the instrument, bluegrass fiddling is mostly done on a violin, it's the way they play it! 

Once uttered by the great Texan Hank Hill, true fans understand that when you're playing bluegrass... you call it fiddle. 

This design was specially made for fans of bluegrass & fiddlers. It reads "When It's Bluegrass, it's Fiddle", with f-stop style swashes and a violin design.