This is a device that attaches to the peg of a double bass to extend the low E string. No modification of the instrument body is required, and it allows you to play notes lower than E. I am selling it because I no longer use it after switching to a five-string double bass. It is in beautiful condition with no scratches or damage. I used it with Belcanto strings, dropping the tuning to around H to C. Depending on the string tension and the bridge setup, it may only go down to D.