Piano wire, sometimes called music wire, is made of steel. When pianos were first invented, piano wire was made of brass or iron.
By the 19th century, piano makers realized that steel worked better than iron, so higher grade steel was being used for the wire by the 20th century. As of 2015, most pianos use ASTM A228 steel music wire.
When a piano is out of tune, the strings will need to have the proper tension reset to make it sound on pitch. Pianos are tuned by using a tuning pin that adjusts the amount of tension in each piano wire.
Today, music wire is also used in a number of different products, such as orthodontic wire, cheese cutters and fishing lures.