Each D'Addario Glass Slide is individually heat treated to provide smooth edges and a perfect finish allowing the slide to glide over strings while the 4mm glass thickness produces a warm, thick tone. These slides are available in small, medium and large sizes. Features : Made from boron silicate for enhanced durability, smoothness and tone Flawless finish and extra smooth surface glides over strings Ring Sizes Medium - 12 4mm wall thickness produces warm, thick tone and sustain 60mm length, 21.5 inner diameter