slack-key

Slack-key guitar (from Hawaiian kī hōʻalu, which means "loosen the [tuning] key") is a fingerstyle genre of guitar music that originated in Hawaii after Portuguese cowboys introduced Spanish guitars there in the late 19th century. The Hawaiians did not embrace the tuning of the traditional Spanish guitars they encountered. They re-tuned the guitars to sound a chord (now called an "open tuning") and developed their own style of playing, not using a flat pick, but plucking the strings. Most slack-key tunings can be achieved by starting with a guitar in standard tuning and detuning or "slacking" one or more of the strings until the six strings form a single chord, frequently G major.
In the early 20th century, the steel guitar and the ukulele gained wide popularity in the mainland, but the slack-key style remained a folk tradition of family entertainment for Hawaiians until about the 1960s and 1970s during the second Hawaiian renaissance. Devotees of the slack-key guitar style tout the alluring resonant sound of the open lower strings juxtaposed with melodies played on the higher strings. Players of the genre typically use pull-offs, hammer-ons, and string harmonics as techniques.
There are many different tunings for slack-key guitar, and the tunings were historically guarded fiercely as a trade secret and often passed down in families.

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