AQUILA ( アキーラ ) AQ-BU THUNDERGUT Bass Ukulele Strings Developed using a high-density, highly elastic material, "Thundergut" bass ukulele strings offer superior performance compared to other polyurethane or silicone strings. They provide a comfortable playing experience, without the excessive friction of silicone rubber that can cause sticking, nor being too slippery. I purchased these and installed them on a ukulele bass once, but I quickly switched to other strings, so they are practically unused. The original box is missing, so I will ship them in a "ThunderBlack" box, but the contents are "ThunderGut".