Kala U-Bass and Aquila Corde team up to bring you this alternative U-Bass string set. Using Aquila's patented technology and formulas, they are engineered specifically for the Kala U-Bass. Their special Nylgut compound includes calcium carbonate to give them increased density, stability and the ability to resonate tone at very low frequencies. They have pure bass, clear tone and a low end that lives up to the name. Significantly different than the Kala black polyurethane U-Bass set. Stock string that comes on the Rumbler U-Bass. Thet are a dark gray color.