These strap retainers are really great for an autoharp. I was given an autoharp to try, but found it way to heavy as when it rested on my arm the weight of the instrument made it difficult to hit the right chord buttons.  I got a D'Aigle slider strap to use for the autoharp. The round objects attach to the strap and you lift up the autoharp, which has the screws placed one per side. ( Be sure to read instructions well, so the screws are in the exact correct location. ) The screws click into the round objects, and the whole unit keeps the autoharp up higher without hurting your arms. For myself, I had to shorten the slider strap on the sewing machine to make it the right size for me. It feels very secure & you can tell it is not going to fall apart & let the autoharp hit the floor. If not for the slider strap and the strap retainers, I would not have even tried to play the autoharp. I see that people use these on guitars as well. You do have to drill holes, so obviously if you did not want holes in the instrument this would not be for you.