This is a rosin spreader from Cecilia, an American rosin manufacturer. In the West, using a rosin spreader to evenly distribute rosin on the bow hair is becoming a trend, not just applying rosin. Depending on bow pressure and playing style, the entire surface of the bow hair may not always contact the strings. By using a rosin spreader to evenly spread the rosin to the back of the hair, you can achieve stable contact regardless of playing style. If you're interested, please search for "Rosin spreader." Although originally developed to evenly distribute rosin for violins, violas, and cellos, it is also effective for double basses. When using high-viscosity rosin, such as for pop music, clumps can form on the bow hair. However, the rosin spreader can eliminate these clumps without damaging the bow hair, allowing the rosin to be dispersed evenly. Rosin is not included.