Tin-plated high-carbon steel with no winding. Ball End.  
D'Addario Helicore strings have a multi-stranded steel core, which results in optimal playability and a clear, warm tone.

Features:

• Multi-stranded steel core.
• Optimal playability whilst producing a clear, warm tone. 
• Versatile string that works well for a wide variety of playing styles and instruments. 
• Suitable for beginner and intermediate students. 
• Small string diameter provides quick bow response. 
• Excellent pitch stability and longevity.
• Packaged in uniquely-designed sealed pouches providing unparalleled protection from the elements that cause corrosion.