Yoon Jeong Heo (허윤정) is a South Korean player of the geomungo, the six-string Korean zither, and above all a master of traditional Korean music. Her name is printed Yoon Jeong Heo on the Black String albums for the German label ACT and on the live album Jazz at Berlin Philharmonic X - East-West, Heo Yoon Jeong on her 2006 solo CD, Heo Yoon-jeong on several of her other credits and in Korean press, and 허윤정 on her Korean ensemble records. She belongs on a jazz site for a specific and limited reason: she leads Black String, a geomungo-centred quartet signed to a jazz label, and she led the Tori Ensemble, a Korean and New York group that a Smithsonian concert page billed as a Korean and American jazz excursion. Most of her own recordings, though, are traditional sanjo and new works for Korean instruments, and this profile says so plainly.

She came to Korean music as a child through traditional dance. Her father was a pioneer of Korean theatre, and at his suggestion she entered the National High School of Korean Traditional Music, where she was introduced to the geomungo. She earned a bachelor's and a master's degree at Seoul National University and in 1984 began studying geomungo sanjo with Han Gap-deuk (Han Gap Duk), a Living National Treasure whose school of geomungo sanjo is Important Intangible Cultural Property No. 16, earning the master title yisuja in his school. From 1990 to 1994 she was deputy concertmaster of the Seoul Metropolitan Korean Music Orchestra. She has performed with the KBS Korean Traditional Music Orchestra and the San Jose Chamber Orchestra, toured Europe, the United States and Asia, and, according to a 2007 New York listing, gave a duo concert with the German composer and multi-instrumentalist Stephan Micus. An Asian Cultural Council fellowship supported a residency in New York in 2007 and 2008. Concert biographies also name her as a member of the soloist ensemble Sangsang, art director of the Bukchon Changwoo Theater, leader of the project group go-MOON-go, and a lecturer at Jungang and Yongin universities. She is a professor of Korean traditional music at Seoul National University.

Her recorded catalogue starts in the traditional repertoire. Young Sanjo Vol. 2 (Woongjin Music, dated 2000 on Discogs, reissued by CJ Music in 2006) pairs her with four other players on one sanjo each: Lee Ji-young on gayageum, Kim Jae-yeong on piri, Seo Yeong-ho on ajaeng and Kim Cheong-man on janggu, with Heo's geomungo sanjo running 16:12. Young Sanjo Live (CJ Music, 2006) has four tracks, a sanjo ensemble, a danso sanjo, a geomungo sanjo and an ajaeng duet. Discogs files both under Jazz and Free Improvisation as database tags, but the track lists are sanjo. Her solo CD Geomungo Solo - Seven Perspectives (C&L Music CNLR 0626-2, 2006) has seven pieces, among them "The Recitation of Pleasure in Zen," "Underplot" and "Geomungo Dance." She plays geomungo on tracks 14 to 17 of the gayageum master Byungki Hwang's Gayageum Masterpieces Vol. 5 (C&L Music, 2007), and her pieces appear on the Korea Arts Management Service compilations Into the Light (2008 and 2011) and on a Warner Classics and Jazz compilation, Anywhere on This Road (2010). She is credited as producer of 2010 Experimental Stage for New Gugak Performance (AkdangEban, 2011) by the all-female modern pansori troupe Byeol-ak. Apple Music also carries a 2020 digital album of Han Gap-deuk school sanjo and other pieces, Geomungo Renditions Heo Yoonjeong, on Mirrorball Music.

The jazz-adjacent work began in 2007. The Tori Ensemble, which Heo led as artistic director, paired three traditional Korean musicians with three New York improvisers: Heo on geomungo, Kang Kwon-soon on vocals and gong, Min Young-chi on daegeum and janggu, and, from New York, the cellist Erik Friedlander, the clarinettist and shakuhachi player Ned Rothenberg and the percussionist Satoshi Takeishi. Its suite Five Directions, in five movements named for the directions and their colours, was recorded live at the Freer Gallery of Art in Washington on June 5, 2007, and performed at Asia Society in New York in December 2008. A CD of the same title was issued by Hub Music (SBCD-21002), billed to the Tori Project, with a note tying it to the Asia Society concert of December 6, 2008. The group returned to New York with The Five Directions of Arirang at Merkin Concert Hall on November 22, 2009 and with an Arirang programme at Roulette in Brooklyn on December 9 and 10, 2011. A separate Hub Music EP, The Willow Is... (SBCD-21038), credits the Tori Ensemble with Heo, Kang, Min and the piri player Lee Suk-joo, with no New York players listed.

Black String came in 2011, and it is the part of her work most often heard abroad. According to an Arts Desk interview, the idea came from the producer Myung-guk Kye during a Korea and UK cultural exchange programme; the first lineup mixed Korean and British players and debuted in London, and once she was back in Korea Heo built the present four-piece around the geomungo. In 2016 it became, by Korean press accounts, the first Asian group signed by ACT. Mask Dance (ACT 9036-2) was recorded in Seoul on June 30, 2016 and released on October 14, 2016, with Heo credited as producer and as composer, with the band, of six of its seven pieces. In February 2017 it won the Best Performance prize in the Jazz and Crossover field at the 14th Korean Music Awards, and in 2018 the Asia and Pacific category of the Songlines Music Awards. Karma (ACT 9045-2) followed on September 27, 2019 with the guitarist Nguyên Lê as guest on two tracks; Black String was one of the acts at the Berlin Philharmonie on November 20, 2019, recorded for the live album Jazz at Berlin Philharmonic X - East-West (ACT 9913-2, released May 29, 2020); and the third album, Road of Oasis, came out on Mirrorball Music on December 20, 2024. Her bandmates are Jean Oh on guitar and electronics, Aram Lee on daegeum, yanggeum and danso, and Min Wang Hwang on janggu and percussion. The group's own profile has the full story.

Other cross-genre projects are documented in smaller pieces. K-Music Encounters I, a streamed concert of November 3, 2020 from the Korean Cultural Centre UK, was billed as a pairing of her with the jazz accordionist Vincent Peirani, with Heo also talking about her musical path. "Auraji River," her duet with the Norwegian trumpeter Mathias Eick, is on Arirang: The Name of Korean Vol. 8 (2021), a set of six Arirang pieces issued by the Korean Traditional Performing Arts Foundation with Youn Sun Nah as music director and the bassist Wonsool Lee as co-producer; Peirani and the saxophonist Andy Sheppard are on other tracks. On March 3 to 5, 2017 she presented Geomungo Space at the Daehakro Arts Theater in Seoul, which combined traditional music, media art and contemporary dance with projection mapping onto a structure of hundreds of strings. In February 2023 she received the grand prize at the first Seoul Art Award for Akgaakmu-Jeolgyeong, a traditional performance staged at the National Theater of Korea in 2022, which the jury said balanced creation and preservation. On November 21, 2023 she played a geomungo and bagpipes concert with Cassandre Balbar at Durham University, and on May 18, 2024 she was billed with the pianist Park Jong-hwa and the British composer and electronic musician Gabriel Prokofiev for tHinG, a geomungo and electronics project, at the Salle Gaveau in Paris; the project's first phase had taken place in Korea in 2023.

Her foundation is the Han Gap-deuk school of geomungo sanjo, the solo instrumental form played with janggu accompaniment. In the Seoul Shinmun in 2017 she said the improvisational quality of Korean music resembles jazz and that she wanted to extend the genre through experiment. In the Arts Desk interview she said Black String's improvisation and experimentation overlap with jazz but that she prefers to call the music "free music that transcends boundaries," and that her aim for the geomungo is to show its dynamic, powerful side, against its image as a masculine, slow and introspective instrument.

Whether she counts as a jazz musician is a fair question, and the evidence points both ways. For: Black String's first two albums are on a jazz label and its Korean Music Awards were given in the Jazz and Crossover field; the Tori Ensemble was built around New York jazz and new-music improvisers and was promoted as jazz-inspired; she has been paired with Peirani and Eick and shared the Berlin Philharmonie stage with Majid Bekkas and Nguyên Lê; and Discogs lists several of her releases with jazz and free-improvisation style tags. Against: she is a classically trained gugak master and a professor of Korean traditional music, her own named records (the Young Sanjo discs and the solo CD) are traditional sanjo, her 2023 grand prize came for a traditional performance, and she describes her own band as beyond category rather than as jazz. The Subgenres above (Ethno Jazz, Crossover Jazz) describe only the cross-genre side of her work. She is best placed as a traditional master with a significant border-zone career.