The names of pitches are associated with particular frequency values. Our current system is called A440 because the note we call ‘A’ that is in the middle of the piano keyboard has been fixed to have a frequency of 440 Hz. This is entirely arbitrary. We could fix ‘A’ at any frequency, such as 439 or 424; different standards were used in the time of Mozart than today. Some people claim that the precise frequencies affect the overall sound of a musical piece and the sound of instruments. Led Zeppelin, a band popular in the 70s, often tuned their instruments away from the modern A440 standard to give their music an uncommon sound, and perhaps to link it with the European children’s folk songs that inspired many of their compositions. Many purists insist on hearing baroque music on period instruments, both because the instruments have a different sound and because they are designed to play the music in its original tuning standard, something that purists deem important.