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 Mexican cowboys introduced Spanish guitars there in the late 19th century. The Hawaiians, for whatever reason, 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 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. 
In the oral-history account, the style originated from Mexican cowboys in the late 19th century. These paniolo (a Hawaiianization of españoles—"Spaniards") provided guitars, taught the Hawaiians the rudiments of playing, and then left, allowing the Hawaiians to develop the style on their own. Musicologists and historians suggest that the story is more complicated,  but this is the version that is most often offered by Hawaiian musicians. Slack-key guitar adapted to accompany the rhythms of Hawaiian dancing and the harmonic structures of Hawaiian music. The New York Times described the music as "liquid, rippling, and hypnotic".  The style of Hawaiian music that was promoted as a matter of national pride under the reign of King David Kalākaua in the late 19th century combined rhythms from traditional dance meters with imported European forms (for example, military marches), and drew its melodies from chant (mele and oli), hula, Christian hymns (hīmeni), and the popular music brought in by the various peoples who came to the Islands: English-speaking North Americans, Mexicans, Portuguese, Filipinos, Puerto Ricans, Tahitians, and Samoans.
The music did not develop a mainland audience during the Hawaiian music craze of the early 20th century, during which Hawaiian music came to be identified outside the Islands with the steel guitar and the ukulele. Slack key remained private and family entertainment, and it was not even recorded until 1946–47, when Gabby Pahinui cut a series of records that brought the tradition into public view.  During the 1960s and particularly during the Hawaiian Cultural Renaissance of the 1970s, slack key experienced a surge in popularity and came to be seen as one of the most genuine expressions of Hawaiian spirit, principally thanks to Gabby Pahinui, Atta Isaacs, Leonard Kwan, Sonny Chillingworth, Raymond Kāne, and the more modern styles of younger players such as Keola Beamer, his brother Kapono Beamer, Peter Moon, and Haunani Apoliona. During this period, luthiers such as the Guitar and Lute Workshop in Honolulu specialized in the development and manufacture of guitars custom made to order for slack-key performance.
Many currently prominent Hawaiʻi-based players got their starts during the Cultural Renaissance years: Cindy Combs, Ledward Kaapana, George Kahumoku, Jr., his brother Moses Kahumoku, Dennis Kamakahi, Ozzie Kotani, three Pahinui brothers (Bla, Cyril, and Martin), the Emerson Brothers and Owana Salazar. These artists, and slack key in general, have become well known outside Hawaiʻi largely through George Winston's Dancing Cat Records record label, which has most often showcased the music in solo settings.
One indication of slack key's increasing visibility beyond the Islands is that the first four winners of the Grammy Award for Best Hawaiian Music Album were slack key collections: Slack Key Guitar, Volume 2 in 2005,  Masters of Hawaiian Slack Key Guitar, Volume 1 in 2006,  Legends of Hawaiian Slack Key Guitar—Live from Maui  and "Treasures of Hawaiian Slack Key Guitar – Live in Concert from Maui." Players from outside Hawaiʻi have also taken up the tradition, for example, Chet Atkins (who included slack key pieces on two of his albums), Yuki Yamauchi (a student of Raymond Kāne's and an advocate of Hawaiian music in Japan), pianist George Winston, and Canadian Jim "Kimo" West (perhaps better known as guitarist with "Weird Al" Yankovic).
George Winston has identified fifty slack-key tunings.  Some are only commonly used for a single song, or by particular players. Mike McClellan and George Winston have developed similar schemes that organize the tunings by key and type. The chart below follows their categories and naming conventions. The tunings were often passed down in families from generation to generation, and tunings were often guarded a fiercely as any trade secret. 
Kī hōʻalu often uses an alternating-bass pattern, usually played by the thumb on the lower two or three strings of the guitar, while the melody is played on the three or four highest strings, using any number of fingers. Many kī hōʻalu players incorporate various embellishments such as harmonics (chimes), the hammer-on, the pull-off, slides, and damping. Slack key compositions exhibit characteristics from indigenous Hawaiian and imported musical traditions. The vamp or turnaround (a repeated figure, usually at the end of a verse) is descended from the hula tradition, and other harmonic and structural features are descended from hīmeni and from the hula kuʻi encouraged by King David Kalakaua. 
Nearly all slack key requires retuning the guitar strings from the standard EADGBE, and this usually means lowering or "slacking" three or more strings. The result is most often a major chord, although it can also be a major seventh chord, a sixth, or (rarely) a minor. There are examples of slack key played in standard tuning, but the overwhelming majority of recorded examples use altered tunings. The most common slack-key tuning, called " taro patch," makes a G major chord. Starting from the standard EADGBE, the high and low E strings are lowered or "slacked" to D and the fifth string from A down to G, so the notes become DGDGBD. As the chart below shows, there are also major-chord tunings based on C, F, and D.
Another important group of tunings, based on major-seventh chords, is called "wahine". G wahine, for example, starts with taro patch and lowers the third string from G to F♯, making DGDF♯BD. Wahine tunings have their own characteristic vamps (as in, for example, Raymond Kāne's "Punahele" or Gabby Pahinui's 1946 "Hula Medley") and require fretting one or two strings to form a major chord. A third significant group is Mauna Loa tunings, in which the highest pair of strings are a fifth apart: Gabby Pahinui often played in C Mauna Loa, CGEGAE.
|G Major or Taro Patch||D G D G B D|
|G Wahine||D G D F♯ B D|
|D Wahine||D A D F♯ A C♯|
|Open D||D A D F♯ A D|
|C Major or Atta's C||C G E G C E|
|Mauna Loa||C G E G A E|
|C Wahine or Leonard's C||C G D G B D|
|C 6||C G C G A E|
|Old Mauna Loa||C G C G A D|
|Open C||C G C E G C|
|F Wahine||C F C G C E|
|Open F||C F C F A C|
|Double Slack F||C F C E A C|
|(Le) Igi, from Samoa|
- Fox, Margalit (March 5, 2008). "Ray Kane, Master of Slack-Key Guitar, Dies at 82". The New York Times. Retrieved May 14, 2020.
- For example, Elizabeth Tatar, "Slack Key Guitar", in Hawaiian Music and Musicians, ed. George S. Kanahele, University Press of Hawaii, 350–360. ISBN 0-8248-0578-X
- See updated and corrected liner notes to the compilation CD History of Slack Key Guitar, by Harry Soria, Jr., Jay Junker, and George Winston.
- "Slack key wins first Hawaiian Grammy", by Tim Ryan, Honolulu Star-Bulletin, February 14, 2005,
- "'Masters' of the Grammy", by John Burger, Honolulu Star-Bulletin, February 9, 2006
- Derek Pavia (February 12, 2007). "Slack Key Snags Third Hawaiian Grammy". Honolulu Advertiser. Archived from the original on August 7, 2009.
- Winston, George. "Dancing Cat Records Hawaiian Slack Key Information Booklet: Section IVa: Chart of Recorded Tunings (and ones closely related to them)" (PDF). georgewinston.com. Retrieved May 13, 2020.
- Tatar, "The Technique" and "The Chant Tradition" sections of "Slack Key Guitar" in Hawaiian Music and Musicians