Many may be familiar with Spanish guitarist Narciso Yepes (1927 - 1997) who created a 10-string guitar which became his signature. The top 6 strings were tuned in standard tuning, while the bottom four were (working downward) C - Bb - Ab - Gb. He never played these, but they were there purely for sympathetic vibration. It added a lot of depth to his sound, not altogether unlike a "Chorus" effect pedal does with an electric guitar.
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