Piano Strings

After a certain point, increasing the gauge of the strings can cause the strings to become stiff, adversely affecting the sound. At this point (an octave or so below middle C), the strings change from plain steel wire to a steel wire core wound with copper wire. The copper winding increases the mass of the string, without noticeably changing its stiffness. This allows the string to vibrate at a lower pitch without having to dramatically increase its length. In the extreme bass, the strings may have a double winding of copper.

The Piano Deconstructed