Willie Berg invented Stonelined Mutes in 1935. They are among the finest brass instrument mutes available in the world. Stonelined Mutes play perfectly in tune and are easy blowing. Stonelined Mutes deliver the perfect professional sound all the time. Included in the many Stonelined Mute features that allow the outstanding colors of sound is the secret formula used to line each and every Stonelined Mute.