This is a trombone mouthpiece case from DAC. It's designed specifically for the Sa-si series cases. The following is quoted from DAC's website: A soft case that can hold up to three mouthpipes with a length of 25cm. It can be secured inside a Sa-si series case with the Velcro on the back. I bought it, but I never ended up buying multiple mouthpipes, so it's been attached to my Sa-si case unused, haha.