Thank you for viewing. I purchased this online as a cello mute, but it's clearly too large, so I believe it's for a double bass. (Regarding the third photo, the left is this product, and the right is a cello mute.) Please understand that I cannot definitively confirm that this is a double bass mute before purchasing. The diameter is 5.3cm, although this is a rough measurement. If you have any questions, please feel free to comment.