Scordatura
In the late 19th century and 20th century there are examples of composers using scordatura. In his Fourth Symphony Mahler asks the violin soloist (the leader of the orchestra) to play on a violin tuned to Gsharp-D-A-Eflat. The music is describing a poor beggar playing a violin. The violin soloist needs a second instrument for this, so that he can swap quickly back to a normal violin.