1the9 was a South Korean boy band formed through the 2018 survival competition Under Nineteen. The group was composed of 9 members: Yoo Yong-ha, Kim Tae-woo, Lee Seung-hwan, Shin Ye-chan, Kim Jun-seo, Jeon Do-yum, Jung Jin-sung, Jeong Taek-hyeon and Park Sung-won. The group's debuting members were announced on February 9, 2019 and was initially slated to be promoting under PocketDol Studio for 12 months. After delays surrounding the COVID-19 pandemic, the group disbanded in August 2020.

Under Nineteen (Korean: 언더나인틴) was a survival show on Munhwa Broadcasting Corporation for trainees under the age of 19 years old. There were a total of 57 contestants competing to debut as a member of 1THE9. The trainees were put into groups that specializes in vocals, rapping, and performance. The show aired between November 3, 2018 and February 9, 2019.