Scordatura
In the classical period a famous example of scordatura is Mozart’s, "Sinfonia Concertante" for Violin, Viola and Orchestra in which he asks the viola to tune all four strings a semitone higher than usual. This makes it sound louder and brighter so that it matches the brightness of the violin, especially as the violin is playing with normal tuning in the key of E flat, which sounds a soft key (because the key of Eflat hardly uses any “open” strings).