Bass String
If the bass strings were made of plain steel wire, the lowest notes would have a string length of around 25 feet long! Since the piano has to be able to fit into normal-sized living rooms, the designers had to achieve a lower pitch using shorter wires. Getting a lower pitch with shorter wire requires using a larger diameter wire. Unfortunately, if a wire is used that has too large of a diameter, the wire will break under the required tension. The solution is to use a smaller steel core wire and then to use another wire wrapped around the core wire to add mass. The extra mass makes the wire behave as though it is a larger diameter wire without causing the problems of string breakage.
Originally, the wire that was used to wrap around the core wire was made of iron. Later, some pianos were made with aluminum-wrapped bass strings, and now copper wrapping is used. Plain steel music wire is used throughout the tenor and treble sections of the piano.
Steel String
A common question is "Why does the piano have only one or two wires for each note in the bass section and three wires in the rest of the piano?" The short answer is that the number of wires used for each note helps determine the volume of that note. A large bass string can produce much more volume than a smaller plain wire. The volume is balanced by using more wires for each note in the treble section and fewer wires for each note in the bass section. 
