Nickel-iron alloy round wound. The outer winding is compressed to an oval shape just before being wound on the string. Crisp and brilliant sound with a smoother than round wound feel. Silk wrapped at both ends to protect your valuable hardware. For five and six string basses add a low 'B' and/or a high 'C' to mix with any of the four string sets listed. Click Compression Wound Singles for single strings.