scordatura explained
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The technique of deliberately mistuning one or more strings of a stringed instrument to achieve unusual sonorities or facilitate difficult passages
Scordatura was common in Baroque music, where violinists, lutenists, and viol players regularly retuned to access different keys, double-stop combinations, and tonal colours. Biber's Mystery Sonatas use a different scordatura for almost every movement, creating fifteen distinct timbral worlds. In modern music, composers use scordatura for microtonal effects and to extend the instrument's range. Guitarists frequently use drop-D tuning, the most common surviving scordatura in popular music.
Biber's Mystery Sonata No. 11 requires the violinist to cross the two middle strings, creating a tuning that allows chords physically impossible in standard tuning.